Methods op improving the stability
of interconnected power systems

ABSTRACT

CONTROL SYSTEMS AND METHODS OF CONTROL FOR CHANGING THE AMOUNT OF POWER GENERATED AND THE MAGNITUDE OF CONNECTED LOAD AND FOR INFLUENCING THE DISTRIBUTION OF POWER FLOW WITHIN (AN INTERCONNECTED) A POWER SYSTEM OR AN INTERCONNECTION OF SYSTEMS THROUGH THE EXECUTION OF CHARGES IN THE DRIVING POWER OF PRIME MOVERS, AND CHANGES IN CONNECTED ELECTRICAL LOADS, WHEREIN INITIATION OF CONTROL ACTION IS RESPONSIVE TO SUDDENLY OCCURRING EVENTS ADAPTED TO CAUSE, OR WHICH COULD CAUSE, SYSTEM INSTABILITY, AND THE POWER FLOW CHANGES TAKE PLACE IN SUCH A WAY AND WITH SUFFICIENT SPEED AS TO PREVENT OR OPPOSE DEVELOPMENT OF INSTABILITY.

Dec. 18, H K RC. METHODS OF IMPROVING THE STABILITY OF INTERCONNECTEDPOWER SYSTEMS Original Filed Feb. 7 1966 SYSTEM 4- SYSTEM 3 SYSTEM 2 wWT J w SYSTEMi L Y mm ADV TT M 5 u mmumi S w R a M c S. R Ts 1Q. T Tr ndR W T511 Q am. new q n1; n-r .M a Q NT T L m w g a g n G mmn i L H EmmEdm T TNT SE AN YR M o S G W C SYSTEM 1 a.

INYENTOR. FIG-\ ROBERT H-PARK BYW,J-- J- RELRY AND CONTROL J\ SYSTEM A TR NETS United States Patent Int. Cl. H02j 3/38 US. Cl. 307-52 23 ClaimsMatter enclosed in heavy brackets If] appears in the original patent butforms no part of this reissue specification; matter printed in italicsindicates the additions made by reissue.

ABSTRACT OF THE DISCLOSURE Control systems and methods of control forchanging the amount of power generated and the magnitude of connectedload and for influencing the distribution of power flow within [aninterconnected] a power system or an interconnection of systems throughthe execution of changes in the driving power of prime movers, andchanges in connected electrical loads, wherein initiation of controlaction is responsive to suddenly occurring events adapted to cause, orwhich could cause, system instability, and the power flow changes takeplace in such a way and with sufficient speed as to prevent or opposedevelopment of instability.

CROSS-REFEREN CE T O RELA TED IN VEN T IONS My invention relates tomeans for rapidly controlling power flow within power transmissionelements of large interconnected power systems with a view to favorablyaffecting the stability of such systems.

More particularly, my invention relates to means, as above, whichrepresent an advance over the prior art including my issued U.S. Pats.Nos. 3,051,842 and 3,234,- 397 the second of which has been reissued asPat. No. Re. 26,571, having to do with employment of line faultinitiated combined use of fast momentary and sustained prime moverdriving power reduction and momentary application of braking loads inpower exporting areas of systems, and also line fault initiated fastboosting [or] of driving power and shedding of load in power receivingareas of systems, as a means of stability improvement and this patent isa continuation-in-part of those patents.

BACKGROUND OF THE INVENTION (1) Field of invention The area of theutility of the invention comprises prevention of development of systemstability within power systems or interconnections of systems whencontinuance of stable operation is threatened by line faults, and otherevents which cause sudden either momentary or sustained interruption orchange of power flow over one or more circuits of the systems powertransmission network.

The area of method comprises employment of means of eflecting changes inthe driving power of generator prime movers, and in the amount ofconnected load, with provision to accomplish within a period of timeshort enough to influence first-swing stability, which, in practice,means within a period of 0.4 to at most 2 seconds.

Thus the field of invention comprises employment of rapid changes inprime mover driving power and connected load, as a way to combat loss ofsynchronism of power system generators such as tends to take place inthe case of certain types of suddenly occurring events.

Among system stability endangering events, the most commonly occurringis a line fault taking the form of a short circuit to ground of one ormore line phases while the most serious fault is a three phase shortcircuit occurring near to a generator or switching station high voltagebus, since, until clear, a three phase fault of this type entirelyinterrupts power transfer over all lines that unite at the bus. A twophase to ground fault tends to have much the same though not quite assevere an efiect.

In power transmission systems a short circuit fault on a line normallyoperates circuit breakers at each end of the line thereby to isolate thefault, while further after a preset period, chosen long enough normallyto extinguish an arc, the breakers may be reclosed automatically.

In some cases circuit breaker reclosure is first efiected from one endof a transmission circuit, as preferably from the end remote from thefault, and later from the other end, except that this second reclosureis only effected if the fault does not redevelop.

In some cases it is elected to reclose breakers in response to amanually, rather than automatically, initiated reclose signal.

During the period prior to the opening of the breakers there is arelative acceleration between those synchronous machines within thesystem which are adjacent to the end of the line that was supplyingpower prior to the fault and those adjacent to the other end of theline, the relative acceleration being such as to increase relativeangular displacement.

The effect of such acceleration at one end, and of deceleration at theother end, of a faulted line is to produce a difference in thevelocities and an increase in the relative generator rotor displacementangle between the group of machines adjacent one end of the line, andthe group of machines adjacent the other end of the line, with aresultant tendency to loss of synchronism and instability which whenoccurring causes voltage disturbances which operate to affect adverselythe performance of connected equipment throughout the system, and insome cases leads to cascading type shut down of generators andwidespread interruption of power supply to ultimate users, and it is amain aspect of the field of invention with which the present applicationis concerned to reduce to a minimum such difierences in velocities andincreases in phase angle.

According to known techniques, design of power transmission systems hasbeen based mainly on the practices of utilizing two or more transmissioncircuits as a way to unite one or more generators, located withingenerating stations with generators and lines located in the balance ofthe system. Usually high voltage busses are provided as a way toelectrically interconnect the generators located at any one station, andalso usually transmission circuits that terminate at a station alsoconnect to one or more busses. Further in the case of long parallellines, one or more sectionalizing busses may be provided at intermediatepoints between line ends.

High speed circuit breakers, and relaying means adapted to rapidly causethem to open are provided at either end of individual transmissioncircuits and are arranged to open when the circuit they control hasbecome faulted thereby to isolate the fault, while also it is often thepractice to reclose the same breakers after a short interval duringwhich the arc at the point of fault may be expected to have becomeextinguished.

With the development of improved methods of coo-ling generators, andimproved designs of the last stages of steam turbines, together with useof high steam pressures and temperatures, it has become possible toprogressively get more and more capacity out of a turbine-generatorwithout need for a proportional increase in inertia of rotating parts.

Also in some cases hydroelectric stations have been located at suchgreat distances from loads that the severity of the problem of providingto preserve system stability despite line faults has increasedconsiderably.

At the same time the present day importance of avoiding even occasionaldisruptions of user power supply has led to general acceptance of a needto design power systems so that loss of system stability does not occureven in the event of delay in clearance of a line fault.

These several factors acting together have resulted in the building ofmore lines, and of lines of higher voltage than would otherwise beneeded (42, 43) but the expense that this has entailed, plus the presentday requirement of taking care to avoid unneeded line construction inorder to minimize adverse effects on the environment, has stimulatedpower industry examination of alternate ways of solving the systemstability problem, and have, in particular, led to a consideration ofthe benefits of line fault initiated fast steam turbine driving powerreduction and momentary application of braking loads (30, 40, 41, 42,43, 44) as ways to preserve system stability, and it has also becomegeneral practice to provide under-frequency initiated load shedding as away to contend with system separations occurring due to loss ofstability or for any other reason.

The field of invention involved in employment of these procedures forthe purposes cited can also be viewed as an aspect of a broader field ofinvention which I have termed fast load contra (41), with a view todistinguishing truly fast acting controls from the almost universallyemployed but slow acting feed-back type meth ads for control of powersystem generated and transmitted loads (45).

(2) Prior art In relation to prior art concepts, it was already old inthe art, prior to the filing of my issued patents, to employ generatortrip ofl", tie line opening, momentary application of braking loads,and, fast momentary reduction of prime mover driving power, as measuresfor improving system stability in the event of a transmission linefault, and also to control such means in relation to both load faultseverity and to conditions obtaining prior to the fault (2, 3, 4, 5, 6,l2) where, as understood in the prior art (2, 4, 6), the word fast meansfast enough to influence generator first forward swing in synchronousspace following fault occurrence.

Early patents that are of interest as examples of prior art compriseU.S. Pat. Nos. 1,705,688, 1,935,292, and 1,834,807 which issuedrespectively to S. A. Staege on Mar. 19, 1929, S. B. Griscom et al. onNov. 14, 1933, and W. F. Skeats on Feb. I, 1931.

The disclosure contained in the Staege patent covers a process ofmomentarily closing a valve that controls the supply of water to aPelton wheel type prime mover of a hydroelectric installation, inresponse to the occurrence of a line fault as evidenced by abnormallyhigh generator current, with. provision to do so rapidly enough toprevent the pulling out of step of the stations generators fromgenerators located in the balance of a power systems generatingstations.

In the area of prime mover driving power control the disclosure of theGriscom et al. patent adds the feature of providing a fast actingby-pass valve so arranged that when the valve of the Staege patentcloses rapidly the bypass valve opens by an amount sufiicient to preventdevelopment of undue hydraulic head in the pentstock that supplies waterto the Pelton wheel.

In addition the Griscom et al. patent discloses the concepts of (a)momentarily applying a resistive load at the generator terminals inresponse to a quick acting line fault responsive control system.

[ Numbers in parentheses refer to appended table of references-II (b)responding rapidly to other aspects of a line fault than currentmagnitude, such as suddenly occurring reduction in either generatorpower output or voltage, and the development of a negative phasesequence component of line voltage.

(c) providing a sluggish power flow responsive device which operates torender inoperative the quick acting fault responsive controls as in (b)above, or in Staege, except when the power exceeds a predeterminedamount which implies inoperativeness if prefault power falls below apreset value.

The Skeats patent added the features of utilizing a polyphase torquemotor as a fast acting fault responsive device, and of varying themagnitude of the resistance load in response to prefault load.

In U.S. Pat. No. 2,285,203 which issued in June 2, 1942, to F. A.Hamilton, Jr. disclosed the idea of providing to maintain synchronousoperation of a pair of power systems that were interconnected by asingle tie circuit, despite a fault on that circuit, by first clearingthe fault, then rapidly reclosing the line circuit breakers while at thesame time applying a: braking load for a lveriod terminating at aboutthe instant of breaker reclosure.

In the area of reduction to practicel the capability of momentaryclosure of both control and intercepting valves to improve systemstability was successfully demonstrated in tests carried out in 1929-30on a 50,000 kw. reheat type steam turbine, with dumping of valveactuator oil employed as a means of achieving sufliciently rapid valveclosure (2), but although European preliminary studies (6, 38, 39),represent an aspect of the prior art, US. interest in fast prime moverdriving lpower reduction as a means of improving system stability failedto revive until after presentation of a 1966 American Power Conferencepaper (41), following which a number of U.S, public utilities initiatedprograms of providing for employment of fast momentary interceptingvalve closing in response to a sudden reduction in generator load suchas takes place when a line fault occurs, while, in one case, provisionwas made to allow simultaneous employment of fast closure of control andintercepting valves (46).

Whereas, as noted in reference 41, over the years there have beennumerous studies bearing on the utility of braking resistors, and, also,it has been reported that there has been working use in the U.S.S.R.,the best known example of operative employment is in British Columbia(30).

As noted in my discussion of reference 42, braking resistors while, inprinciple, capable of functioning effectively as a system stabilityimproving device, cost much more than does provision for momentary fastvalving, and for this reason, for the present at least, their field ofutility appears to be of limited to hydro-electric installations.

[My first issued] In 1956 I filed a patent application which dealtexclusively with the use of braking resistors. Subsequently afterenlargement of scope to include fast prime mover driving power controlUS. Pat. No. 3,051,- 842 was granted. This patent dealt in part withline fault responsive procedures for electrically isolating a generator,generating station, or a generating segment or area of power system,from the balance of the system applying braking and fast prime moverdriving power reduction during the period of isolation, as a way toreduce generator acceleration and thereafter reconnecting, to thebalance of the system.

In another aspect, my first issued patent dealt with the use of fastprime mover driving power reduction and braking applied in a generatingsegment or area of a system on the occurrence of a fault on a linecomprising only one path of two or more electrically parallel paths ofpower flow leading from the generating area to a power receiving area ofa power system and with no generators or elements of the system otherthan the faulted line itself isolated.

An aspect of my first issued patent was that fast prime mover drivingpower reduction was optionally executed, not merely momentarily, as inthe prior art, but also on a sustained basis.

Another aspect of that patent was that when isolation was employed, fastdriving power reduction and momentary application of braking load wereoptionally used in combination, whereas, when isolation was not used,these measures were proposed for use only as alternative procedures.

A further aspect of that patent was the concept of modulation of linefault initiated prime mover driving power reduction responsive totransmitted load, through employment of a control system which alteredits control characteristic in response to transmitted load up to theinstant of fault, but that was uninfluenced by what happened during orafter the fault.

My second US. Pat. No. 3,234,397, introduced the concept of employingline fault initiated momentary plus optionally sustained type primemover driving power reduction apply in combination with dynamic brakingwithin a generating segment or power exporting area of a power system inthe multiple parallel path of power flow case in which generator orsystem element isolation would not be used, with a view to reducinggenerator acceleration within the generating segment and added twoadditional concepts, namely fault initiated at least momentary loadshedding, optionally supplemented by fault initiated boosting of primemover driving power both measures being executed in power receivingareas of power systems as a way of reducing deceleration of generatorslocated in such areas and added the principle of response of prime moverdriving power and connected load controls to the occurrence ornon-occurrence or redevelopment of a line fault, when and if faultedline breakers were automatically reclosed.

Further, my second US. patent deals with the case wherein a generatingstation is tied to a power receiving system over parallel lines, and thepower receiving system has a weak tie to a second power system, and itis an object of the fault initiated fast prime mover driving power andelectrical load control system to not only prevent loss of synchronismof the generating station with the system that receives power from itdirectly, but to also avoid loss of synchronism of that system with thepower system to which it is only weakly tied.

Thus, my two issued patents dealt with situations in which a fault on aline connecting two elements of a three element interconnection couldcause loss of synchronism of any one element with the other two.

It was an aspect of both my two issued patents, and also acharacteristic of prior art patent and literature references to providemeans of avoiding development of instability of the generators of agenerating station or generating segment or area of a power system whena fault occurred on one or more essentially radial transmission lineslinking the generating station or generating segment of a system to apower receiving system, where the receiving system was viewed asessentially unitary, which is to say tending to behave dynamically as asingle entity.

However, it was also an aspect of both of my issued patents to providemeans of dealing with situations wherein a generating station or a powersystem or area of a system containing generating capacity is joined to apower system interconnection by lines which makes connection overdisparate paths of power flow from the station, system or system elementto dynamically independent components of the interconnection.

Thus, my issued patents show three power systems or segments or areas ofa system, which contain generators and which are interconnected by threelines, and it is an aspect of both patents that control action initiatedin response to a line fault is modulated as to extent and nature inresponse to the weighted sum of the power that is being transmittedprior to the fault over pairs of lines which represent disparate pathsof inter or intra systems Also, beginning in 1962, means not shown in mypatents, or other patent literature, were described which alford auseful way to control fault initiated application of braking load andprime mover driving power, as also generation rejection, all with a viewto prevention of system instability, which incorporate response toconditions obtaining subsequent to a disturbance (14, 18, 27).

Further, it has been proposed to employ rapid variations of thepotential applied to grids of grid controlled A.C.-D.C. converters asmeans of rapidly changing power transfer to, from, or withininterconnected power systems over high voltage direct current lines, asa way to counter tendencies to system instability when line faults occuron alternating current transmission lines of the interconnection (25).

Aside from what is described in my second patent, I know of only threeprior or: approaches to what could be termed event, as against drop infrequency, initiated load shedding, as follows:

(a) load shedding [carried out within a system that loses all connectionwith another system, and directed to avoiding] responsive to theoccurrence of reestablishment of a fault on automatic reclosure offaulted line circuit break rs foll wing a fault on one of a group of twoor more lines uniting an area of a system to the balance thereofemployed as a way to retain synchronism and avoid a system separation(36).

(b) load shedding eifected via radio signals responsive to the event ofa generator trip off, and directed to avoiding a system separation (37).

(c) load shedding responsive to the event f loss of [one but not alllines] a single line uniting an area of a system to the balance thereofand employed as a way to prevent drop in frequency following separation(23). These references, it may be noted, do not cite event initiatedcombined load shedding and prime mover driving power boosting.

Prior to the issuance of my first and the filming of my second potentthere was filed in 1961 in the name of M. A. Eggenberger a US. patentapplicati n which disclosed an overspced anticipation device taking theform of a fast acting means for effecting a closure of a turbin e'scontrol valves in response to a predetermined degree of deficiency ofgenerator load relative to driving power, which application eventualedin U.S. Pat. N 3,198,954, issued Aug. 3, I965.

The basic concept of the patent was that a sudden drop in load wouldcause the device to initiate valve closing with a minimum of delay.

In the period following presentation of reference (40) a modified formof this device in which control action takes place in response to thequantity P-L where P represents turbine driving power as evaluated witha steam pressure transducer and L represents electrical load as measuredwith a Hall Effect watt transducer has been provided as a feature of asubstantial number of turbinegenerators furnished to customers of thetwo U.S. producers of large steam turbines-generators, as a way toinitiate fast operation of intercepting valve actuator-oil dump valvesand thereby initiate rapid momentary intercepting valve closure where toimprove power system stability when threatened by line faults (47).

On Aug. 29, 1967, thus subsequent to the filling date of the parentpatent of this reissue application, there was filed in the name of S. P.Moorganov, a patent application which represented a continuation of anapplication filed on May 8th, 1963, and which evcntuated as US. Pat. No.3,421,014, issued Jan. 7, 1969.

This patent resembles the present reissure application and its parentpatent to the extent that it covers methods for very rapidly varying thedriving power of power system steam turbine type generator prime moversin response to the occurrence of events which operate to endanger systemstability such as line faults and the momentary or sustained opening ofinter or intra system time lines, and which could cause, or have caused,a system separation. Also, as in the present application and its parent,the Moorganov patent introduces the concept of employing anunderfrequency responsive ralay as a way to temporarily "increase therotational standby power power of the power system owing to a partiallift on the power rise limitations (column 4, line 47, 48), whichpresumably could be rephrased as a way to temporarily admit more steamto the turbine than could be safely admitted on a long term basis, andby this means boost turbine driving power with the purpose of minimizingfrequency drop if and when a system separation takes place.

However, the means of accomplishment proposed for use in Moorganovdiflers materially from that disclosed in the present application andparent patent in that in Moorganov the aproach is to employ a controlchannel to ensure control of the power of said prime mover underemergency conditions, said channel having means for forming primarycontrol signals, feed back means connected to the input of said primarycontrol signal means, compensating for said primary control signalsdepending on the influence applied by said control system to said primermover (cf. column 30, lines 32 through 39).

In the above quotation the words "compensate for" would appear torequire interpretation, since they do not appear to conform to U.S.usage, and it is assumed that in such usage it would be proper tosubstitute the terms "which are arranged to modify."

In contrast it is a feature of the present reissue application and ofits parent patent as well as of U.S. Pat. No. 3,305,842 and Re. 26,571of which it is a continuation in part that, while the control channelused to ensure control of the power of the prime mover under emergencyconditions is fitted with means for forming primary control signals, thefurther feature of feed back means connected to the input of saidprimary control means" which are arranged to modify "said primarycontrol signals depending on the influence of said control system tosaid prime mover" is not employed.

Rather in the present application and my issued patents the concept isto provide a turbins control system with a preprogrammed signal orseries of signals, without provision of other than conventional feedback response to turbine behavior, with fast acting control efiectedentirely on an open loop or feed forward basis with the nature of thesignal determined in part by the nature of the system disturbing event,for example as to whether a line fault or a generator power output orsustained tie line power flow interruption due to reasons other than afault, and if a fault, in response to the occurrence or nonoccurrence ofa refault on faulted line reclosure following fault clearance (Re.26,571) and, additionally on the basis of response to prefault stationload and the magnitude and direction of transmission circuit loadings.

As it would seem, what has been stated just above can be phrased in theterminology of the Moorganov patent by stating that in my applicationand patents I employ what is referred to in the Moorganov potent as"discrete action systems cf. column 3, lines 3, II, 33).

However it is to be emphasized that my discrete action systems arearranged to respond to initial preemergency conditions, while by virtueof the rior art as exemplified by Griscom et al., the concept ofproviding so that control system response has relation to the severityof a fault would appear to be fully open to the public.

In a sense it appears that in the areas of intended use that are common,what Moorganov ofiers comprises feed back control features thatsupplement, and, in the inventors view, represent improvements over whatis shown in Griscom et al., in my patents and in the Eggenberger patent.

Thus, there has been very substantial [recognition] prior artconsideration of the need for and means of providing what I hereindesignate generally as fast load control" [possibilities] procedures.

SUMMARY OF THE INVENTION Though there has been prior use of what Itermed fast load control, heretofore utilized or proposed fast [load]prime mover driving power control systems which repond on an open loopor feed forward basis to the occurrence of events, such as line faults,that operate to endanger system stability, including those shown in mytwo issued patents, have been directed to improving the stability ofpower systems, whether large or small, under circumstances wherein theobjective was to entirely prevent loss of synchronism and sustainedinterruption of power flow between any system power generating and powerusing elements of a power system in the event of a fault.

In contrast, the present invention is directed to favorably affectingthe stability of large interconnected power systems by employment offast prime mover driving power [and system connected electrical load]control means and procedures which are activated in response to eventssuch as trip off of an individual generator for any cause, the trip offof all the generators in a power station due to the pulling out of stepof the station generation as a whole, opening of a system tie lineresulting in overloading of parallel paths of power transfer, whether ornot resulting from a line fault, loss of a radially connected load or aradial connection to a power source which had been feeding power to aninterconnection, and the termination or a. sudden decrease or increasein power transfer over a D.C. line as a result of A.C.-D.C. convertercontrol action, or other events, which cause or could cause theoccurrence of sustained power flow interruptions involving either majorchanges in the amount of power transferred between two portions of aninterconnected power system, or

[By way of illustration, the present invention provides a way to dealwith] sustained interruptions of power flow over a group of transmissionlines comprising at least two but not all lines of a power transmissionsystem which unite a portion of the interconnection to the balancethereof by at least three lines.

Also in this, my invention offers a means of favorably affecting systemstability [in the event of] within an area of a system that separatesfrom the balance of a system or within a system or area of system thatseparates from an interconnection of systems due to lack of success inentire maintenance of system stability on execution of a line faultclearance.

In this connection, it is an aspect of my invention to set forth a novelway of modulating employment of fast load control used in the aboveconnections, as in response to predisturbance values of system powerflows.

Again, an aspect of my invention relates to provision of improved meansof accomplishing prime mover driving power control and load sheddingoperations.

Thus, with reference to prime mover driving power control, most powersystem steam turbines which employ fossil fired steam generators are ofthe reheat type, and in relation to this point, are subject tolimitations as to feasible speed of accomplishment of driving powerchanges, and it is a part of this invention to show how theselimitations can be in considerable degree overcome by modified turbinedesign.

In addition, the present invention introduces the concept of fast,system disturbance initiated, momentary load shedding, together withpreselected load type, short duration, sustained load sheddingincorporating provision for progressive, time delayed, automaticreconnection, wherein controls, located in customers premises, andacting in response to power supply interruption applied to selected loadareas and maintained for a controlled period which may comprise only afraction of a second, or responsive to other signals, operate todeenergize selected load elements on a sustained basis, and thereafterautomatically cause progressive reenergization over selected timeperiods, which can be varied as between customers to afford timediversity, and also adjusted to allow time for activation of systemspinning reserve or quick start-up nonspinning reserve and/ orrearrangement of system loads via changes in load-frequency controlsystem settings.

To aid in making clear certain aspects of my invention, it may bedesirable to bring out the point that, whereas employment of fastcontrol of steam turbine driving power has been made use of by the powerindustry, over the years, it appears to have been applied only to nonreheat turbine generators, and also always employed in only threeconnections, i.e.

(a) to neutralize cyclic tie line power swings (8, 29, 32)

(b) to rapidly boost the power generation of a power receiving system inthe event of development of a condition of reduced system frequency.

(c) to minimize power swings over tie lines which unite power systems tosteel rolling mills where the mill generates power with use of a nonreheat-type steam turbine (29).

Under modern conditions small systems commonly are joined to form largersystems, large systems are joined with other systems to form pools, andpools are joined together to form superpools or large interconnections,with the effect that presently, for example in the U.S. east of theRockies, about 130,000 megawatts of generating capacity are usually tiedtogether (31), from which it follows that until synchronism is lostwithin the system, even the tripping oil of a 1,000 megawatt generatorwill have little effect on frequency.

Thus, under conditions which we will designate as conditions applying toan extended interconnection," the effect is that ordinary systemdisturbances do not alter frequency, appreciably which means that in theevent of a system disturbance, speed governors play almost no part untilthey are eventually influenced by slowly acting, conventionalload-frequency control equipment (31).

Accordingly, in an extended interconnection, any equipment for boostingturbine driving power that operates rapidly only in response to a systemfrequency drop will have no opportunity to function rapidly until andunless a portion of the system either pulls out of step, or isdisconnected by tie line circut breaker action.

Thus, under extended interconnection conditions currently prevailing inthe U.S., if a generator is tripped off the line in, say, theConsolidated Edison system, when that system is tied to its neighbors,and until such time as loadfrequently controls have come into effect,every generator in the U.S. east of the Rockies that is not alreadyfully loaded will try to contribute its proportional share to make upfor the loss in power supply that results, which implies that for allpractical purposes a surge or inrush of power nearly equal to the outputof the [operator] generator that was tripped will [flow into] be.Yuperim posed on pre-trip values of power flowing over ConsolidatedEdisons [via its] ties to neighbors (24A).

Also, in the case of certain systems wherein presently non reheatturbine standby generators are normally run at low output, subject toautomatic, fast driving power boost in the event of a frequency drop,the effect is or will tend to become that the turbines in question willfail to boost output in the event of a sudden loss of system generatingcapacity, except only in the event of a disturbance which causes systemisolation, since frequency drop due to even a large loss in capacitywill be small, and therefore difiicult to distinguish from frequencychanges that occur at times when fast output boosting of standbyequipment would not be desirable.

In contrast, the fast load control features of the present inventionprovide means to rapidly activate system spinning reserve within a largeinterconnection and [the] simultaneously shed load rapidly, as also toapply electrical braking load and fast prime mover driving powerreducingtion at other points of the system, optionally in combinationwith fast changes in power flow over D.C. transmission systems executedin response to A.C.-D.C. converter control signals which may beprogrammed to come into effect independently of frequency.

In respect to the load shedding aspect of the present invention, it maybe noted that whereas the position of utilities has tended to be thatload shedding should not be planned except by contract arrangement.However, in view of difficulties of the Nov. 9, 1965, U.S. northeastblackout, it may eventually be judged to be in the public interest thatin the future power companies plan on more general brief automatic loadshedding in the event of stringent contingencies.

In this connection, preselected load type, short duration load sheddingoffers a way to alleviate objections to load shedding that have hithertoprevailed, while also it will be easily recognized that such provisionfor load shedding would operate to afford advantages relative to easeand feasible speed of restoration of system voltage following anoccasion of total voltage collapse.

With respect to the aspect of the present invention having to do withability to rapidly boost output of reheat turbines, it may be noted thatsuch capability can reduce need for load shedding, and also provides, away to allow compensating for the appreciable delay in driving powerbuild up in response to governor control point biasing that normallyapplies to other than low head water driven prime movers and to gas andnon reheat type steam turbines.

It is an [aspect] object of my invention [that it] to descri'be[s] newways to use known fast load control devices and combinations of suchdevices such as described in my issued U.S. Pats. No. 3,051,842,3,234,397, and Re. 26,571 as aids to preservation of system stabilitywhereby to enhance the utility of such devices.

[It is an] Another object of my invention is to provide a way of makingeffective use of [any appropriate type of] fast [load] acting primemover driving power means and combinations of such means with fastacting connected electrical load control means [control procedure] as anaid to preservation of the stability of an extended power systeminterconnection on the occasion of an event which either causesisolation of a power supplying or power accepting element of theinterconnection or involves the interruption of power flow over at leasttwo lines of a power transmission system which unites a portion of theinterconnection to the balance thereof by at least three lines.

Another [aspect] object of my invention is to provide [a useful]improved ways of modulating employment of fast acting prime moverdriving power and connected electrical load control means in extendedpower system interconnections in the event of a disturbance [comprising]involving a system power flow interruption wherein modulation isresponsive to predisturbance values of system power flows.

Also, a further, related object of my invention is to provide controlmeans, independent of system frequency, for initiating power systemprime mover driving power boosting and load shedding in the event ofdevelopment of a local power supply deficiency within a power system.

Another related object of my invention is to provide a means of fastboosting of driving power of prime movers, and fast load shedding whichis adapted for use in power systems which represent components of anextended system interconnection.

Another object of my invention is to provide ways for improving thecapabilities of power system reheat type steam turbines to execute rapidboosting of prime mover driving power.

Another object of my invention is to provide within an extended powersystem interconnection a system of momentary load shedding together withsustained, preselected load type shedding of load within customerspremises, coupled with provision to automatically reenergizeprogressively over a controlled time period.

Another object of the present invention is to provide an improved way bymeans of which both steam turbine and waterwheel driven generators maybe utilized as source of power system spinning reserve generation.

Another object of my invention is to provide improved means of [fastload control including but not limited to employment of] fast primemover driving power control and/or fast momentary and/or preselectedload type, short duration load shedding which are adapted to usefullysupplement methods that have heretofore been proposed and/or used forimproving the ability of [such] power systems to resist development ofsystem instability.

The subject matter which is regarded as the invention is particularlypointed out and claimed in the concluding portion of the specification.The invention, however, both as to organization and method of practice,together with further objects and advantages thereof, may best beunderstood by reference to the following description taken in connectionwith the accompanying drawings in which:

DESCRIPTION OF DRAWING FIG. 1 is a diagrammatic representation of aninterconnection of four power systems which is adapted to be operatedwith the present information.

FIG. 2 is a diagrammatic representation of a control system adapted tofavorably regulate fast load control means.

FIG. 3 comprises a curve showing a functional relationship.

FIG. 4 comprises a memory unit.

FIG. 5 comprises a load sheding and load shedding control system.

FIG. 6 comprises a modification of the load shedding system.

FIG. 7 comprises a reheat type steam turbine and an associated steamgenerator.

FIG. 8 is a control system diagram.

FIG. 9 is a system diagram showing provisions for fast activation ofspinning reserve.

FIG. 10 is a further system diagram showing provisions for fastactuation of spinning reserve.

DETAILED DESCRIPTION OF THE INVENTION Referring now to FIG. 1, system 1is shown connected to each of systems 2, 3 and 4 by power transmissionsystems 1-2, 1-3, and 1-4, which, though shown as alternating currentdual lines, could also be single lines, or a group of more than twolines, or could comprise or include a direct current system tie or ties,while also where some lines are shown of equal voltage, and some ofdifferent voltage, all could be of the same, or all of differentvoltages.

Systems 2, 3 and 4 are intended to represent elements of a large powerpool or extended interconnection of systems, and are shown connected toeach other by transmission systems 2-3 and 3-4 and also may be furtherinterconnected, as through other lines of the interconnection of systemsin which they are participants, yet may typically be dynamicallyrelatively independent, or integral," in that generator rotors of eachsystem can be expected to swing relatively in the event of a fault oneither of transmission systems 1-2, 1-3 or 1-4.

Power is generated within system 1 by generators G to G driven by primemovers PM, to PM Line and generator circuit breakers shownconventionally are to be understood as controlled by relay systems 1, 1aand 1b, which are to be understood as arranged to be responsive tosystem currents and voltages and to also be controlled in response tomanual tripping, as also to the operation of or control by a number ofautomatic breaker trip initiation means that are commonly provided witha view to preventing catastrophic type boiler, turbine, and generatorfailures that could otherwise occur in the event of operatingdifficulties due to failure of equipment components (34, 35).

Watt transducers (WT) (WT) etc. are shown connected to currenttransformers, and also receive voltage inputs from voltage sources notshown, in such manner as to be responsive to generator power outputs,and total power flow over transmission systems 1-1a, 1-2, 1-3 and 1-4.

Also it is to be understood that relay system and watt transduceroutputs are supplied as inputs to control systems 1, la and 1b, whichincorporate various fast system load control means.

Elements BR; to BR comprise generator braking resistor installations.

Dotted lines show paths of control.

It will be recognized by those skilled in the art that relay systems 1,1a and 1b will normally incorporate a selection among variousconventionally used (19) line fault, line overload, and out of step, andgenerator, trans former, and bus relays.

It will also be recognized that the operation of such relays can be usedvariously, alone or in conjunction with one or more already known typesof power responsive or other controls (3, 4, 12, l4, 16, 18, 27) toprogram application of electrical braking load, fast boosting andbucking of prime mover driving power, and fast load shedding.

Thus, in particular, my issued and pending patents teach measures whichcan be employed to prevent loss of synchronism of generators G and Gwith the balance of the system in the event of a fault on a line oftransmission system l-la or equally of generator G in the event of afault on transmission systems l-lb, or of system 1 with the balance ofthe interconnection in the event of faults on transmission systems l-2,1-3 or 1-4, while also the same teachings have application in case ofemployment of DC. transmission systems.

However, the concepts of the present invention go further in that, forexample, they are directed to make provision to favorably influencesystem stability when, subsequent to a fault on a line of transmissionsystem 1-1a, or in response to any other initiating event, as forexample the pull out of generators G and G followed by out of steprelaying, a sustained and entire interruption of power flow overtransmission system l-la occurs, a fast load control system responsiveto the initiating event is activated and caused to operate in a manmeradapted to minimize the power inrush through transmission systems l-2,1-3 and 1-4 which tends to develop with loss of the power output ofgenerators G and 6,.

Also, the same concepts apply in relation to events which leads tosustained cut oil" of, or in the case of DC. lines, to any majorsustained reduction of power flow, over any of transmission systemsl-la, l-lb, 1-2, 1-3 or 1-4, or to the sustained termination of powersupply by any system 1 generator as in relation to the tripping of agenerator output circuit breaker, or otherwise.

In general, control effects needed will depend on various factors. Thus,the opening of the output breaker of a 600 mw. system 1 generator mightwell represent a less adverse event than the interruption oftransmission system 1-3 power fiow under a load of 600 mw.

For example, if system 1 were receiving 600 mw. over transmission system1-2 and say mw. over each of systems 1-3 and 1-4, which, in thisinstance, were onehalf the voltage of transmission system 1-2, in theabsence of fast load control the effect of interruption of power flowover transmission system 1-2 would be that 13 the power supplied overtransmission systems 1-3 and 1-4 would tend to increase to a total of900 mw. corresponding to a 2 to 1 increase in load or to 3 times theoriginal value, which might be much more than enough to cause system 1to pull out of step.

In contrast, in the case in question, loss of 600 mw. of system 1generating capacity would tend to boost total power flow overtransmission systems 1-2, 1-3 and 1-4 from 900 to 1500 mw. whichcomprises an increase in load to of the original value. Hence the amountof generator prime mover boosting and load shedding if any needed topreserve stability would differ markedly.

Also if a system 1 generator breaker were tripped open at no, or onlylight, generator load, or if power flow over any of transmission systems1-la, l-lb, 1-2, 1-3, or 1-4 were interrupted, for any reason, at onlylight load, load shedding and/or driving power boosting would not beneeded, but rather would be undesirable.

Again, tripping off of a generator or interruption of power flow overtransmission systems l-la or 1-1b under heavy load could be unimportantif it occurred at a condition of light load on transmission systems 1-2,1-3, and 1-4.

Accordingly, a system disturbance initiated, power responsive control isneeded which controls differently in relation to the nature of the eventor events, as also system conditions, with which the disturbance isassociated, thus as to whether in relation to interruption of power flowover lines of transmission systems l-la, 1-1b, 1-2, 1-3 or 1-4, or tothe opening of output circuits of generators 6,, G G G or G as also inrelation to power limits and predisturbance power flows overtransmission systems 12, 13, and 1-4.

In this connection, the effect of a system disturbance will sometimes[relates to] give rise to hazard of or cause instability within a powerreceiving system or area of a system or interconnection of system inview of develop ment of a deficiency of [a system] prime mover drivingpower, [therefore tending] within the receiving system, or area, theefieet being to retard space phase of prime movers, [and consequentlycall] which, in turn implies a need for fast prime mover driving powerboosting and/or system load shedding, and, where D.C. tie lines areinvolved, for boosting of power flow toward or reducing power flow awayfrom the system, while in other cases there will be an excess of primemover driving power, therefore calling for fast prime mover drivingpower reduction, and/or electrical braking load application andgeneration rejection, and also boosting of power flow away from orreduction of power flow toward the system over D.C. lines.

Now, in this connection, if P =system 1 export power=power input tosystem 1 bus 1 from all sources other than such of transmission systems1-2, 1-3 and 1-4 as [one] are of the AC. type, less the system 1 bus 1load and if P P and P =input to system 1 bus 1 over such of transmissionsystems 1-2, 1-3 and 1-4 respectively, as are of the AC. type, it willfollow that under all conditions so that if P, were suddenly altered byan amount AP it would follow that such change would be matched by suddenchanges in P P and P AP AP; and AP which would necessarily conform tothe relation or say there would be At this point consider the casewherein the system is operating under steady load conditions, whensuddenly a portion of system 1 generated power is lost due to thetripping off of a generator, or in the event that input of power tosystem 1 bus over transmission systems l-la 14 or 11b is suddenlyinterrupted, or in the case of a D.C. line, perhaps merely reduced.

Evidently, in such case, in an extended interconnection whereinappreciable frequency changes cannot occur, and with maintenance ofsynchronism, and subsequent to a period within which system generatorrotors decelerate or accelerate, to take new relative positions, andalso load is shed and prime mover driving power boosted, and/or D.C.line power flow toward bus 1 increased, it will tend to be true that lm= 1= aJ 234= 1 a -sb where:

AP =amount of system power input cut-off sb= s+ b (5 L,,=amount of loadshed in system 1 P =a measure of benefit equivalent to load sheddingthat can be derived from as fast as feasible system 1 prime moverdriving power boost plus effects due to fast regulation of power supplyover D.C. lines if any although Equation 4 will not follow entirely, asthere can be a drop in system voltage which can act to somewhat decreasesystem load.

However, in any case, the relationship of Equation 4 comprises a goodgeneral guide for use in relation to programming fast load shedding andprime mover driving power boosting.

Also, in relation to such of transmission systems 1-2, 1-3 and 1-4 asare of AC. type, there will be a maximum limit of power flow towardsystem 1 bus that can be transmitted on a sustained load basis over thesystems in question say P without development of instability, whilealso, as see FIG. 12 of reference (1), the maximum surge power AP thatcan be transmitted 111 a given direction over a single line initiallycarrying a load P in the same direction, and where the power P =value ofPg34=P2+P3 prior to the disturbance and K is a numerical factor that canbe determined by system studies, test, or expenance, and would includeconsideration of steady state as well as momentary power limit, butwhich can have a value approximating .73.

Accordingly, to avoid loss of synchronism, it is necessary that sb g234( 234m 23l-o) (8) which can be taken to imply that to avoid hazard ofinstability P should be so adjusted that sb= s 234 aam- P2340) aa-tmwher; k is suitably chosen as say in the range of 0.1 to 0.

. the other hand, in the general case where P2340 is either positive ornegative, it is necessary to take account of the fact that where P2340can be negative, Equation 6 has to be written in the form AP=P f(P /Pwhere f(x) is a function such that, in the range x=0 to where 1 butwhich bends over for negative values of 1: generally as shown in FIG. 3and has the value f(x)=1.0 at x= 1. Accordingly, it follows that anappropriate relation for P in the general case of P either positive ornegative should be sought in the form sb r 234m Z34 334o 234111) 234mwhere f (P /P; can be found by means of calculation or by test orexperience, but will tend to resemble fi(x) as shown in FIG. 3.

If now power flow over transmission system l2 were to be interruptedsuddenly, and assuming that at least one of transmission systems 1-3 and1-4 is of the AC. type, it would be equally in order to expect arelationship to hold of the form and correspondingly in the event ofinterruption of power flow over lines 1-3 and 1-4.

Now, with respect to interruptions to power flow over transmissionsystems 1-la, 1-1b, 1-2, 1-3, and 1-4, it is recognized that in theevent of line faults, and where these transmission systems incorporatemore than one line, it will normally result that a fault develops ononly one line, and that line alone is isolated, and also usuallyisolated only momentarily except when the fault is of the permanenttype, while ways of maintaining system stability in the event of thistype of system dis- ;urbance have been extensively studied and providedAccordingly, the present invention has been directed to dealing withthose situations which have not heretofore been effectively dealt with,wherein as a result of any event, power fiow over all lines of thetransmission system is interrupted, as can occur as a result of out ofstep type (19, 33 or other (27) relay action, whether or not for goodcause.

Accordingly, with this concept there is a need for a system that willdevelop control signal voltages generally as given by Equations 12 and13 for use in modulating the operation of load shedding and prime moverdriving power boosting means.

FIG. 2 comprises such a system.

Thus, referring to FIG. 2, memory units 1 through are connected toreceive as a component of their voltage inputs the outputs of all powertransducers located in the system 1 high voltage bus area, and also[telemetering] telemetered transducer outputs of generators G G and Gshown under designations (WT) Ym and na- In addition, memory units Mthrough M receive supplementary voltage inputs comprising DC. voltagesource V which is made suitably proportional to -P plus the sum of theoutputs of transmission systems 12 and 1-3 and 1-4 power transducers,except reduced in the ratio K to l, by means of a dropping resistorhaving branches (K flr and (lK )r and modified by the effect ofrectifier RECT, plus the voltage of DC. voltage source V; which is madeproportional to 'i ZMm- FIG. 4 shows that memory unit M consists of aquick acting relay 5 wherein normally closed contacts make a path toallow charging a condenser C through a resistor R by supply of voltageto terminals 7 and 9, while when coil 11 of the relay is energized, thecircuit through R is interrupted and the terminal of the condenseradjacent to R is connected to terminal 13 which in FIG. 2 is connectedto the lead to terminal 1, while terminal 7 is connected to the lead toterminal 3.

Returning now to FIG. 2, proper operation requires the selectiveactivation of memory devices by sustained energization of coil 11 of theappropriate device, as per the schedule following:

16 Power interruption: Memory unit Generator 6, output 1 Generator 6;output 2 Generator 6;, output [5] 3 Generator 6.; output [6] 4 GeneratorG output [7] 5 Transmission system 1-la (all lines) [3] 6 Transmissionsystem l-lb (all lines) [4] 7 Transmission system 1-2 (all lines) 8Transmission system 1-3 (all lines) 9 Transmission system 1-4 (alllines) 10 More than one method of eifecting desired selectiveenergization of memory units can be used. For example, in the case ofgenerator circuits it would normally be desirable to energize memoryunit relays from a suitable power source via a normally open latchingtype relay which closes and latches-in when the generator output circuitbreaker trip circuit is energized.

However, at the sacrifice of a loss of speed of control system response,it would also be possible to energize through a latch-in type relayarranged to be energized by generator breaker auxiliary contacts whichclose when the breaker opens.

Again, it would be possible to energize in response to the operation ofa control device which becomes operative in anticipation of a need, oronly a possible need, to trip the generator breaker, and as part of acycle in which fast load control is applied and the generator breakertripped, if at all, only after a delay. For example, a. power swing typerelay (27) could be employed as a means of anticipating the possibiltyof development of generator pull out.

Also, generally similar considerations apply to energization of relaysof transmission system memory units except that provision needs to bemade so that energization of the relay of the appropriate memory unit isonly accomplished when entire power flow over all lines of a radialtransmission system such as 1-1a, or 1-lb, is interrupted, and whenentire power flow over all lines of selected groups of all lines unitingsystem 1 to the balance of the interconnection is interrupted.

In FIGS. 1 and 2 the group of all lines uniting system 1 with theinterconnections that were selected comprised.

Transmission system Description All lines uniting systems 1 and 2. Alllines uniting systems 1 and 2. All lines uniting systems 1 and 4.

However, other groupings could have been used.

For and merely by way of example, if group 1 above comprised twoparallel or double circuit lines uniting system 1 and system 2 over onepath, and two other parallel or double circuit lines doing so over adifferent path, it could be considered desirable to add sub groups tothe above table, as below Transmission system Description Group:

15 1-2 Two lines following path a. ll) 1-2 Two lines following path b.

the combination of transmission systems 1-2, 13, and 1-4, which operateto unit system 1 to the interconnection.

Accordingly, if control in response to power flow over line groups laand 1b were to be made a part of the fast load control system, it wouldyet be desirable to retain power flow over all group 1 lines as in FIG.2 signal generation control means, and to add two more memory units 11and 12 which would be arranged to receive appropriate watt transducerinputs.

As regards details of how to energize memory units, where response is tobe had in relation to opening of all of a selected group of linebreakers, for example all the breakers of each transmission systems1-1a, 1-1b, 1-2, 1-3, or 1-4, a suitable procedure would be to provideeach line breaker with a latch-in relay which would close its contactson a sustained basis when the breaker trip coil was energized, and toconnect the contact circuits of the relays corresponding to each groupin series.

However, where a group, such as group 1, has sub groups, such as subgroups 1a and 1b, it would also be necessary to provide so that when alllines of group 1 were tripped, a relay would be energized which openedthe circuit to coils 11 of the sub group memory units, and only acted toenergize the group 1 memory unit after a delay sufiicient to allow thesub group memory unit relays to drop back to their non-energizedpositions, or alternately some equivalent system, as with use ofrectifiers, whereby to avoid discharging the condenser of group 1 memoryunit into a sub group memory unit condenser.

Again, it would be possible to energize transmission system memory unitsother than in response to energization of breaker trip circuits, and, inparticular, provision to activate fast load control in response to sometype of relay, or other information source, that would give advancenotice of a probability of a system, as say system 1 or 2, pulling outof step with the interconnection would often be desirable.

Accordingly, it is a part of my invention that the option may beexercised of providing for activation of memory units other than inresponse to line breaker tripping, and accordingly, with a view torapidly initiating and controlling type and amount of fast load control,by means which respond to an event which could or might be expected tocause a power interruption. Thus, to be specific, memory units could beactivated in advance of energization of line breaker trip coils by useof power swing relays (2, 3, 4, 27), voltage dip relay (3, 4)acceleration detection relays (6), generator momentary speed responsivecontrol (1 8), generator vs. system displacement angle units 14), andcertainly other types of relays or controls, as also in response to apurely manually aplied signal.

At this point it is proposed to return to consideration of the specialcase of interruption of generator power output in response to an outputcircuit breaker trip signal, and to the way in which the circuits ofFIGS. 4 and 2 cooperate.

Thus, evidently, if resistor R is large relative to resistor r, as is tobe understood has been provided, and if generator 1 receives an outputbreaker trip signal during an otherwise undisturbed system condition,and if memory unit M relay acts faster than the operation of power flowinterruption, which would also be provided for, memory unit M, willmemorize and develop between signal generator output terminals 1 and 3 avoltage proportional er Gl 234( 234m 234o)+ aasm up to the point where K(P -P )=P which occurs when P -(lK ,,)P while in view of rectifier RECT,for more negative values of P the voltage will be simply Relations (14)and together provide a rough approximation to Equation 12, and are onthe safe side.

Also, similar results will apply in the event of tripping oil ofgenerators G to G and also with interruption of power flow overtransmission systems l-la and l-lb.

Also, by further reference to FIG. 2., it will be clear that suddeninterruption of power flow over transmission systems 1-2 from an initialsteady load condition will produce values of P corresponding to amodified form of equation 13 up to the points respectively that K (P l)=l and beyond this point to a value of P,

Also, similar results, but with an appropriate change of notation, wouldapply to interruption of power flow over transmission systems 1-3 and1-4.

Again if, for example, transmission system 1-2 represented a D.C. line,Equations 14 and 15 would need to be modified by replacing K 1 and P byK P and P while Equations 16 and 17 would still hold, except that ifpower flow over the DC. line toward bus 1 were merely reduced by anamount AP instead of wholly cut off, AP; would replace P in Equations 16and 17.

To obtain desirable performance in respect to signal generation, accounthas to be taken of the fact that if generator breaker tripping or linefault interruption occurred only subsquent to a line fault followed byline fault clearing with reclosing, and perhaps with use of fast loadcontrol including application of braking resistors, fast prime moverdriving power control, and load shedding, the transducer output thatacted to determine P would be subject to variations due to dynamicphenomena related to generator rotor oscillations, and by virtue of thisfact would, in general, fail to operate to determine average value ofpower in rush over transmission systems 1-2, 1-3, and 1-4, and thetherefore it could be desirable that R and C be so chosen that thequantity RC representing the condenser charge-discharge time constantwas large enough to at least partially mask such effects.

In practice, best value of RC could be determined by suitable systemstudies carried out in application to situations or in relation toclasses of cases, for example as between what would best apply inrelation to opening of generator output circuits, and what totransmission system power flow interruptions, and, especially inrelation to the case of transmission systems the matter of naturalperiod of dynamically integral system elements, united by the system,and adapted to be set in relative oscillation, as by fault clearanceprograms that might ultimately result in the opening of the breakers ofall transmission system circuits.

In the light of these considerations it would appear preliminarily thatresults of studies might be expected to show that suitable values of RCwould lie generally in the range to 10 seconds.

It will be noted that it has been necessary in what went before to makea distinction between transmission systems such as systems 1-2, 1-3 and1-4 which tie system 1 to the interconnection, and transmission systemssuch as systems 1-1a and l-lb that do not.

This distinction would apply and hold true independently of the numberof transmission systems involved of each type.

In the event of connection of system 1 to the interconnection over Ntransmission systems, the interruption of power flow over any one systemwould be handled by the same type of circuit as shown in FIG. 2 but withappropriately modified parameter values as also designations. Thus, asto designation, the power limit designations applying either to loss ofa generator in or interruption of power flow over a radial transmisisonsystem in system 1 could be written without ambiguity simply as K, P,and P while coefiicient K and power and power limit applying in theevent of interruption of a group, say group g,, of lines connecting aportion of a system interconnection to the balance of theinterconnection could be written without ambiguity as P K say P and Prespectively.

Thus, in the general case the signals would be, for isolation of powersupply of receiving system element, as per Equations 14 and I5, exceptreplacing K by K and P and P2340 by P and P and for interruption ofpower flow over a group of lines 3 uniting a portion Of aninterconnection to the balance of the interconnection, as per Equations16 and 17 except replacing K by K and am and 340 by umm and rime- It isrecognized that more advanced means of deriving a signal voltage havinga magnitude indicative of need to program desirable extent of fast loadcontrol activity could be devised, and also that further study, orexperience, could and probably will reveal the desirability of takingexplicitly into account, dynamic phenomena such as can arise prior tofull interruption of power flow over a transmission system, andespecially dynamic conditions that can develop during lengthy faultyperiods and processes of going out of step.

While more versatile means of control can be devised, they evidentlycould be delt with as in the nature of adjunct improvements to thesystem herein shown which could be brought into effect in the form ofsupplementary voltages added to the memory unit inputs and outputs ofthe FIG. 2 signal generator. a

With a quantitative signal made available, by whatever means, thereremains a need for a control scheme which will react suitably. Such ascheme is shown in FIG. 5.

Thus, referring to FIG. 5, voltage appearing across terminals 1 and 3 ofthe signal generator of FIG. 2 is shown applied to a circuit comprisingresistor 15, rectifier 17, and the axle end of wiper arm 19 of fastoperating stepping switch 21, and from the contact end of the wiper armto any one of a first bank of stationary contacts 23, and thence back toterminal 3. Amplifier 25 actuates fast relay 27 in response to voltageacross resistor 15, while relay 27 in turn energizes stepping motor 29.

Resistor 31, which is preferably chosen larger than resistor 15, bridgesacross stationary contacts 23, with the result that appearance of morethan a minimum signal voltage across terminals 1 and 3 will cause thestepper to step to a position generally proportional to the signal.

Stepper switch 21 is provided with a second bank of stationary contacts33 which operate in conjunction with a progressive shorting sector wiper35 to apply battery voltage progressively to contacts 33, each or atleast some of which, as here illustrated in relation to the first ofcontacts 33 only is connected so as to control the opening of at leastone system load feeder circuit breaker.

Automatic feeder breaker reclosing is an aspect of the invention. Theprocess of automatic reclosing may be inherent in the breaker on thebasis of a preset but not necessarily uniform reclosing time for eachbreaker, or may be made such that reclosing occurs only when the breakertrip signal is removed with control of instant of removal, as by theaction of one or more time delay circuit opening relays, such as relays39 and 41, which also could be selectively energized from any ofcontacts 33, or otherwise, operated selectively by supplementary fastload control means not herein detailed, and which could also be made tooperate in response to type of system disturbance, as for instancewhether or not involving interruption of power flow over transmissionsystems 1-2 or 1-3, or in response to presence or absence of dynamicphenomena occurring in advance of activation of memory units.

As also shown in FIG. 5, means may be provided in customers premises sothat lighting and/or other load that would be advantageously kept asfully energized as feasible during load shedding, is reenergized as soonas the feeder breaker recloses, whereas other load, such as, in the caseof residences, water heating, electric ranges, refrigerators, oilburners, home heat, etc., is temporarily disconnected as for example bythe opening of contactor 43 with reclosing determined by fast openingtime delay reclosing relay 45.

Evidently, customers load may be subdivided into portions some of whichare reenergized after one time period and some after another, as byproviding additional contactors and relays.

Also, as important to achievement of system diversity factor withrespect to time of pickup of system load subsequent to initiation ofload shedding, closing time of time control relay 45 would preferably bevaried as between customers.

As an alternate fast load shedding method, which could prove useful, asespecially in relation to the shedding of selected portions ofindustrial loads, selective shedding could be accomplished as per FIG. 6wherein a remotely controlled, fast opening, time delay reclosing relay47 controls fast opening contactor 43 in response to signals suppliedover a suitable communication channel such, for example, as a telephonecircuit.

So far what has been shown relates to load shedding.

However, in the event availability of fast load control means in theform of fast generator prime mover driving power boosting capabilityand/or capability of either rapidly boosting system 1 input power orreducing output power over D.C. lines, provision to rapidly implementsuch means either directly in response to power boost signal generatorvoltage, or in response to an associated stepping switch or othersupplementary signal responsive control would be in order, either aloneor supplemented by fast load shedding.

Evidently, a simple means of accomplishment would be to apply FIG. 2signal generator output directly to a suitable voltage responsiveturbine control such as utilized in equipment described in references(29), (8) and (22), which could include phase advance and/or otherantihunting features, or to a suitable voltage responsive D.C.transmission system rectifier or inverter output control unit, asillustrated in FIG. 8.

In this there would be the point that boosting steam turbine output canrequire /2 to 1 second (29, 8), so that in use of turbine outputboosting, supplementary provision for employment of at least momentaryload shedding could often be desirable.

In relation to prime mover driving power boosting, in cases where slowdriving power response waterwheel driven generators are in operation assay at a remote generating plant, at less than full load, and henceavailable as system spinning reserve generation, while also one or morebase load reheat turbines are in operation, there is the point that withprovision for turbine valving downstream of the reheater, the energystored in the turbine reheater as also with availability of valving toallow high pressure turbine bypassing, also the energy stored in thesuperheater can be made use of to provide momentary large turbinedriving power boost for the period of possibly 4 to 10 seconds thatcould be required to bring the waterwheel driven generator unit up tofull output.

Here various means of accomplishment would be feasible, as to which twopossible systems are shown in FIGS. 9 and 10.

Thus, referring to FIG. 9, the steps of the stepper unit of FIG. 5 couldbe used to implement a telemetered boost message to a remote waterwheeldriven generating plant while the local steam turbine unit could besupplied directly with a driving power boost voltage signal which wouldbe cut back after a time period by fast closing time delay opening relay65.

Again, a possibly desirable alternate procedure could be that shown-inFIG. 10 wherein, on say the first step of the stepper relay, both ofdual fast opening valves 49a and 49b open, while after a time delayperiod controlled by fast closing time delay opening relay 67, valve 4%recloses.

Whereas we have up to now dealt with a condition in which a deficiencyof power supply to system 1 develops, it also needs to be recognizedthat the reverse problem can arise, as when system 1 primarily exports,rather than receives power over any of transmission systems 1-1b, 1-2,1-3, or 1-4, in which case there can be a need to apply braking load and[a] reduce system 1 generator prime mover driving power, with a view topreventing loss of synchronism of system 1 with the interconnection.

In this it will be clear that handling of power export conditions can bedealt with by use of precisely the same type of control signaldevelopment circuit shown in FIG. 2 except with exclusion of generatorpower output and transmission system 1-1a circuits, which could have nofunction since these circuits do not absorb load under normalconditions. Also, transducer connections would have to be reversed, i.e.transducer voltages would represent power outputs from system 1 bus,rather than power inputs. Also it will be evident that FIG. could alsobe used to momentarily apply braking load, and to program fast reductionof prime mover driving power, with provision to activate not onlybraking units BR, and BR: of FIG. 1 but also, when desirable, units BRBR, and BiR in response to line carrier or other communication channelsignal, while the arrangement of FIG. 8 would apply in relation to primemover and DC. line power flow control.

In what has gone before there has been reference to problems of rapidlycontrolling driving power of steam turbines.

In this connection, momentary steam turbine driving power reduction byfast valving, though limited in speed by steam entrainment effects, isfacilitated by the fact that turbines are conventionally designed toclose valves rapidly in order to prevent overspeeding on loss of load.

However, the great bulk of power system turbine generators utilizereheat type turbines which, as previously noted, introduces certainproblems. Thus, in relation to fast boosting of reheat turbine drivingpower, delay results in response to opening of the turbine controlvalves by virtue of the fact that it is necessary to increase reheaterpressure before flow to the intermediate and low pressure turbines isinfluenced, and the further fact that due to the volume of the reheater,the time constant of such changes is typically 5 to 20 seconds.

Accordingly, in the case of turbine-generators of present type, it tendsto be true that in starting from an initial stable load condition, withthe intercepting valve already wide open, only the high pressure turbinecan respond rapidly to a process of fast control valve opening.

Beyond the foregoing, there is the further problem of there being a verysubstantial economic incentive to run turbines at their best point,which means with control valves all the way open, in order to achievefavorable fuel economy. This, in turn, tends to imply inability toincrease driving power.

In this, ways of boosting power which can be applied notwithstandingturbine operation at best point that have been recognized (13, 20, 26)and used, include:

(1) Bypassing feedwater heaters (2) Increasing spray water ahead offinal superheater and reheater (3) Bypassing the high pressure turbine.

However, these procedures do not avoid delay in response of theintermediate and low pressure turbines incident to time required toEfill the reheater.

In relation to boosting reheat turbine driving power the presentinvention shows how to speed response by providing either a bypassaround one or more stages of the intermediate pressure turbine,including the first stage, or a variable area nozzle system ahead of thefirst stage wheel of the intermediate pressure turbine, in either casewith the effect that capability of a rapid boost in driving power of theintermediate and low pressure turbines is achieved.

In relation to boosting driving power it is evident that steam stored inthe reheater represents a momentary source of turbine driving powerwhich has the benefit of being immediately available, and which willalso tend to be sustained for a period related to the reheater timeconstant, while with fast response so achieved, boosting on a continuedbasis will be possible by supplementary employment of measures (1) to(3) as above, as also by control valve action in the case of turbinesnot operating at full load, and by utilizing measures adapted toincrease steam generation, such as providing and bringing into use steamgenerator [for] overcapacity [and] by supplementary gas fuel supply, andmeans to heat feedwater with gas.

To accomplish these results, I employ a steam generator and turbinecontrol system as shown in FIG. 7.

Referring now to FIG. 7, the diagrammatic representation containedtherein discloses a steam turbine installation comprising high,intermediate and low pressure turbines with a reheater located betweenthe high and intermediate pressure units, generally as shown in US. Pat.3,055,161 to J. I. Argersinger et al., and wherein common elements areidentified in FIG. 7 with like numbers, except with provision to bypassnot only the high pressure turbine, but also at least one and preferablyseveral stages of the intermediate pressure turbine, beginning at thefirst stage, or even to bypass all stages, with employment of valves 49and 51 in parallel, where valve 49 can represent a quick acting,electrically controlled open or shut type valve, and valve 51 representsa small bleed valve.

Valve 53, which controls application of spray water to desuperheater,55, is in series with water flow control valve 57 which is to beunderstood supplied with a suitable source of spray water.

Valves 49 and 53 are normally closed, but are adapted to open rapidlywhen activated by control unit 59 in response to signals received bysignal generator unit 61, which could comprise respectively the devicesof FIG. 5 and FIG. 2.

With the arrangement as shown, when a turbine driving power boost signalis received, valves 49 and 53 would rapidly open and immediatelyincrease the flow of steam through the intermediate and low pressureturbines by drawing on the stored steam in the reheater.

As this occurs, the reheat pressure would drop, causing an increasedamount of power to be developed in the high pressure turbine, thereby inpart tending to compensate for reduction in pressure of steam availablefor driving the intermediate pressure turbine.

The spray system, complete with bypass water separator, is provided toreduce the temperature of the steam entering at the intermediateadmission point of the intermediate pressure turbine, with a view toreducing turbine rotor and casing thermal stresses.

Valve 51 is held partly open to constantly bleed steam in order to keepthe bypass water separator and the piping hot, while also a similarvalve would be provided around valve 62 in each case in order to preventexcessive condensation efiects when bypassing is suddenly resorted to.

If determined to be desirable, valve 49 could comprise a fast acting,servo operated, control type valve with electrical controls generallysimilar to the intercepting valve shown in U.S. Pat. 3,097,488, M. A.Eggenberger et al. (22) and therefore could be controlled directly fromsignal generator voltage. However, turbine intercepting valve 129 inFIG. 7 would also preferably be of the type in question, therebyallowing controlled rapid reduction in turbine driving power when wantedin response to signal generator voltage, and providing a means of fastmodulation of steam flow to the intermediate pressure turbine whichwould be operative were valve 49 chosen to be of the merely open or shuttype.

Again, as a further and possibly preferably means of accomplishing thebasic objective of providing a way of rapidly boosting the driving powerof reheat type steam turbines, I propose that the nozzles of the firstturbine stage following the reheater be made of variable area, using forthe purpose a construction generally along lines commonly employed ingas turbines produced by the General Electric Co. (U.S.), and alsodescribed in U.S. Pat. 2,651,496, Buckland et a1.

As to details of construction, it will be recognized that prevention ofleakage of steam at high pressure would presumably make necessary ordesirable enclosing the operating mechanism in a pressure tight shellwhich, however, it is judged, would not represent a diflicult technicalproblem.

Also, in relation to provision of variable area nozzles, it will beclear that considerable simplification would be possible over what isshown in the patent reference in that the nozzles would be applied tothe intermediate pressure turbine first stage, while also with provisionfor modulation of intercepting valve opening, it would be possible tooperate the variable nozzle area mechanism as a 2-position system, i.e.,a normal position, and a further open flow boost position.

As pertinent to the invention, there is the point that for turbineswhich use dual reheat, i.e. turbines with reheaters ahead of the secondand third units, each of these units could be provided with bypassvalving, or alternatively with variable area first stage nozzles.

Evidently it would be possible to also employ fast bypassing offeedwater heaters and boosting of superheater and fast application ofreheater sprays as a supplementary means of increasing steamdevelopment.

Further, as a relevant aspect of the invention, there is the point thatit appears feasible to provide so as to allow boosting of output ofsteam generator by 20 or more percent for a period of an hour or less bymerely going to the relatively small capital expense involved inproviding added combustion air fan and fuel supply capacity, whichtherefore often could be desirable especially when it is borne in mindthat under modern r extended interconnection conditions, even subsequentto system disturbances, reserve power requirements of other than shortduration presumably could usually be arranged via power flow over systemties given a period of say 5 minutes to one half hour to allowrearrangement of intersystem power flows.

In respect to water sprays, there is also the point that where fast flowchanges are contemplated, water spray quantity would preferably becorrelated directly with steam flow or steam flow valve opening, ratherthan controlled only in response to temperature as in the Argersingerpatent.

Thus in this connection spray control could be related in the firstinstance and rapidly to steam flow, with added supplementary slow actingtemperature control if wanted.

In the matter of practical means of accomplishment of fast load controlbut which are not deemed to constitute a patentable aspect of thepresent invention, it may be worthy of note that it is possible toemploy certain useful 24 procedures in relation to fast reduction ofprime mover driving power.

For example, in fast valve closing of intercepting valves, there hasbeen expression of concern on the part of some engineers as to chance ofdamage to valve seats if fast closing is too often employed, whilefurther there has been concern as to blowing of high pressure safetyvalves with fast closing of turbine control valves, 1n view of the factthat blowing such valves tends to cause leaking, which, in turn, willcontinue until a complete turbine shutdown period.

However, I propose to avoid these problems by o y partially closing theintercepting valve, in response to employment of servo control, or by atfirst only closing the intercepting valve partially, and thereafterfurther closing it at reduced speed, whereby to avoid mechanical damage,while also I propose to avoid operation of the high pressure safetyvalves by providing fast acting and fully commercially available dumpvalves, which are programmed by the fast turbine control system to dumphigh pressure steam, either to atmosphere, or, preferably, to theturbine condenser, with concomitant supply of spray water for coolingpurposes.

Again, a generally similar bypass to the condenser could always be usedto prevent operation of the reheater safety valves, although it isunderstood that from a practical standpoint advantages derived may beunimportant.

Again, in relation to fast reduction of prime mover driving power, aproblem can arise in that speed of intercepting valve opening iscommonly somewhat restricted with a view to minimizing possible turbineoverspeed hazards.

Where only a brief reduction of driving power is wanted, a problem couldarise if the intercepting valve were fully or nearly closed, in that insome cases delay in reopening would offer disadvantages. However, thisproblem can also be reduced in scope by the procedure of providing so asto only partially close the intercepting valve in response to suitableservo system control.

Also, it will be clear that where a sudden, sustained drop in drivingpower of a reheat type turbine is wanted, it will be only feasible toachieve desired results by using the intercepting valve as a steam flowmodulation device to supplement control valve modulation of steam flowto the high pressure turbine which, however, it is judged can beaccomplished by those skilled in the art merely with application ofgenerally known practice (21, 22).

Throughout, in what has gone before, numbers in parentheses in the texthave been supplied in reference to the table of references below, whichhas been made use of as a convenient means of presentation of pertinenttechnical information already known in the art.

1) System Stability as a Design Problem, R. H. Park and E. H. Bancker,Trans. AIEE January 1929, vol. 48, p. et seq.

(2) Governor Performance During System Disturbances," R. C. Buell, R. I.Caughey, E. M. Hunter, and V. M. Marquis, Trans AIEE March 1931, vol.50, p. 354 et seq.

(3) U.S. Pat. 1,834,807, System of Electrical Distribution," W. Skeats,Dec. 1, 1931.

(4) U.S. Pat. 1,935,292, Regulator System, S. B. Griscom and C. W.Wagner, Nov. 14, 1933.

(5) U.S. Pat. 2,285,203, Control Equipment," F. A. Hamilton, Jr., June2, 1942.

(6) Report of the International Study Committee No. 13: System Stabilityand Voltage Load Frequency Control," B0 6. Rathsman, CIGRE Report 336,1952,

(7) U.S. Pat. 2,651,496, David Buckland et al., Sept. 8, 1953.

(8) Controls for Operation of Steam Turbine-Generator Units," 0. N.Bryant, C. C. Sterrett, D. M. Sauter, Trans AIEE February 1954, vol.73-1IIa, p. 79 et seq.

(9) Load Reduction by Underfrequency Relays During System Emergencies,"W. C. Gierisch, Trans AIEE February 1955, vol. 73 III-B, p. 1651 et seq.

(10) Application and Test of Frequency Relays for Load Shedding, L. L.Fountain and J. L. Blackburn, ibid., p. 1660 et seq.

(11) Automatic Load Shedding," AIEE Committee Report, Trans AIEEDecember 1955, vol. 74, p. 1143 et seq.

(12) Increasing the Reliability of Operation of Power Systems and LongDistance Transmission Lines", D. I. Azaryev, L. G. Mamikonyants, I. A.Syromyatnikov, and A. A. Venikov, CIGRE Report No. 318, 1958.

(13) large Steam Turbine Generators With Spinning Reserve Capacity, J.E. Downs and E. H. Miller, 1960 American Power Conference.

(14) The Use of Repeated Electrical Braking and Unloading To Improve theStability of Power Systems, V. M. Gornshtein and Ya. Luginskii, ElectricTechnology U.S.S.R., vol. 2, 1962, p. 292.

(15) U.S. patent application Ser. No. 219,711, Means for MaintainingStability of Power Transmission Systems Nothwithstanding 3 Fault, R. H.Park, filed Aug. 27, 1962, which issued as U.S. Pat. 3,234,397 on Feb.8, 1966, and was subsequently reissued on Apr. 29, 1966 as U.S. Pat. Re.26,571.

(16) U.S. Pat. 3,051,842, Means for Maintaining Stability of PowerTransmision Systems During a Fault," R. H. Park, Aug. 28, 1962.

(17) U.S. Pat. 3,055,181, Method of Operating a Power Plant System, J.I. Argersinger et al., Sept. 25, 1962.

(18) The Stability of a Hydro-Electric Generator With Electric Braking,D. Ye. Trofimenko, Electric Technology U.S.S.R., vol. 1, 1962, p. 70.

(19) The Art and Science of Protective Relaying," C. Russell Mason, JohnWiley & Sons, Inc., New York, 1962.

(20) "Advisory Committee Report No. 1 on Methods of Carrying Peak Loadsand Methods of Reducing Peak Loads, Federal Power Commission, January1963.

(21) U.S. Pat. 3,097,490, Electro-Hydraulic Control System for TurbineWith Pressure Feedback, P. C. Callan et al., July 16, 1963.

(22) U.S. Pat. 3,097,488, Turbine Control System," M. A. Eggenberger etal., July 16, 1963.

(23) A Selective Load Shedding System," G. D. Rockefeller, WestinghouseEngineer, November 1963.

(24) Coordinated Regional E.H.V. Planning in the Middle Atlantic States,U.S.A.," J. A. Casazza, R. W. Werts, W. B. Fisk, E. S. Van Nostrand,CIGRE Report No. 315, 1964.

(25) Parallel Operation of A-C and DC Power Transmission, Peterson,Reitan and Phadke, 1964 IEEE International Convention Record, p. 84 etseq- See also Trans. IEEE-PAS 1965, pp. 15-19.

(26) "National Power SurveyFederal Power Commissionl964, part 1, pp.125-127, U.S. Government Printing Office.

(27) A Power Swing Relay for Predicting Generation Instability, R. D.Brown and K. R. McClymont, IEEE No. 65-90, Winter Power Meeting, 1965.

(28) Boiler-Turbine Coordination During Startup and Loading of LargeUnits," F. J. Hanzalek and P. G. Ipsen, ASME No. 65-WA/PWR-9, December1965.

(29) 25/41 MW Cyclic Steam Power Plant Serves Hot Strip Steel FinishingMill," D. V. Fawcett, IEEE No. 65-36, Winter Power Meeting, 1965.

(30) "Dynamic Stability of the Peace River Transmission System," H. M.Ellis, A. L. Blythe, J. E. Hardy, and 1. W. Skooglund, IEEE No. 65-813,October 1965.

(31) The Automatic Control of Electric Power in the United States,"Nathan Cohn, IEEE Spectrum, November 1965.

(32) "Fringe Generation Damps Tie-Line Oscillations, Electrical World,Jan. 10, 1966, p. 48 et seq.

(33) A Modern View of Out of Step Relaying, G. D. Rockefeller and W. A.Elmore, IEEE No. 66-34, Winter Power Meeting, 1966.

(34) Five Years Experience on the Consolidated Edison System WithProtection of Turbine Generators and Boilers by Automatic Tripping, W.C. Beattie, H. A. Bauman, J. M. Driscoll, T. J. Onderdonk, R. L. Webb,Trans, AIEE February 9, vol. 77-III (and discussion), pp. 1353-1366.

(35) Protection of Large Steam Turbine Generator Units on TVA System,"M. S. Merritt, J. A. Ackerman, R. C. Price, and L. E. Owen, IEEE PowerApparatus and Systems," April 1965.

(36) "The Story of Bonneville Power, Friedlander, Spectrum, December1968, p. 72.

(37) Letter to the Editor, Peter Kubilius, Electrical World, Nov. 29,1965, p. 5.

(38) Sokolov, N. 1. et al. Elektrichestvo 1963, No. 10, pp. 5-13.

(39) Kashtelan et al., Elektrichestvo', 1965, N0. 4, pp. 1-8.

(40) DeMello, F. P., Ewart. D. N., Temoshok, M., and Eggenberger, M. A.,Turbine Energy Controls Aid in Power System Performance, Proc. AmericanPower Conference 1966, vol. 28, pp. 438-445.

(41) Park, R. H., "Improved Reliability of Bulk Power Supply by FastLoad Control," Proc. Amer. Power Conf., 30, 1128-1141 (1968).

(42) Concordia, C., and Brown, P. G., "Efiects of Trends in Large S teamTurbine Driven Generator Parameters on Power System Stability, IEEEpaper 71 TP74- PWR, Trans. IEEE Power Apparatus and Systems, pp.2211-2218.

(43) Lokay, H. E. and Thoits, P. 0., Effects of Future Turbine-GeneratorCharacteristics on Transient Stability," IEEE paper 71 TP75-PWR, Trans,IEEE Power Apparatus and systems, PAS 90, pp. 2427-2431 (1971),November-December.

(44) Cashing, E. W., Jr., et al., Fast Valving as an Aid to Power SystemTransient Stability and Prompt Resynchronization and Rapid Reload AfterFull Load Rejection," IEEE paper 71 TP705-PWR (see Trans. IEEE PowerApparatus and Systems).

(45) Kirkmayer, Leon K., Economic Control of Interconnected Systems,John Wiley and Sons, 1959.

(46) Farmer, R. 6., Kent, M. H., Hartley, R. H. and Wheeler, L. M.,"Four Corners Project Stability Studies," Paper No. 68 GP 708-PWR,Trans. IEEE Power Apparatus and Systems.

(47) Park, R. H., U.S. Pat. No. 3,657,552, Method of Employment of FastTurbine Valving," Apr. 18, 1972.

(48) Eggenberger, M. A., et al., U.S. Pat. No. 3,198,- 954, "OverspeedAnticipation Device, Aug. 3, 1965.

(49) Giras, Theodore C. and Hofiman, Arthur 6., "High Speed Tie-LineController, Proc. Amer. Power Conf., vol. 27, pp. 898-907 (1965).

(50) Electra-Hydraulic C ntrol for Improved Availability and Operationof Large Steam Turbines, M. Birnbaum and E. G. Noyes, IEEE Conferencepaper 31 CP C5-776.

(51 "Safe Cycling of High Pressure Steam Turbines," Werner Trassl,Proceedings of the American Power Conference, vol. 31, 1969, pp.306-313.

(52) "Managing Megawatts, a Case History," Gordon R. Friedlander, IEEESpectrum, May, 1972, pp. 39-41.

(53) Eggenberger, M. A., Introducti n to the Basic Elements of ControlSystems for Large Steam Turbine- Generators, IEEE Tutorial Course Text70 M29-PWR "The Role of Prime Movers in System Stability, pp. 91-141.

(54) O. I. Aanstad and H. E. Lokay, Fast Valve Control Can lmpriveTurbine-Generator Resp nse to Transient Disturbances," WestinghouseEngineer, July 1970, pp. 114-118.

Whereas in the foregoing citation has been made se eral references,(namely 3, 4, 12, 14, 16, 18, 27), as describing use of already knowntypes of power responsive or other controls as a way "to programapplication of braking load, prime mover driving power boosting andbraking and load shedding, on a feed forward basis the point appliesthat other reference could have been cited as of the date of filing ofthe parent application, namely references 2, 5, 8, 9, 10, l3, 17, I8,21, 22, 29, 30, 33, 36 of the present and parent application. Alsofurther references that could have been cited comprise notably U.S. Pat.No. 3,198,954 which issued to M. A. Eggenberger et al. on Aug. 3, 1965,(48) and two other 1965 publications (49, 50, the first of which (49)describing a fast acting feed-back type turbine control system that culd readily be modified so as to provide still faster open loop feedforward type control action, and the second (50) which also representsitem (d) of reference I of reference 47 of this application, whichdescribes a Westinghouse electrohlydraulic type control system that isgenerally similar to the GE Co. system described in references 22 and53.

Still another significant reference that cites fast acting prime moverdriving power control means that were commercially available pri r tothe date of file of the parent of the present application, comprises a1969 American Power Conference paper by Werner Trassl (51) of SiemensAG. which states that as early as 1960 there began to develop widespreadinstallation on the continent of Europe of power system, steam electricinstallati ns which combined employment of one-through boilers withreheat type steam turbines equipped with electrically controlled meansfor simultaneously closing control and inintercepting valves in secondin response to a feed forward type signal, together with simultaneouslyinitiated opening of steam by-pass valves which become opened in 6seconds.

Also applicant has been advised by engineers of two other Europeanproducers of power system steam turbines, namely M.A.N. and Brown Boverihave, since around 1960, supplied power system turbines and controlssimilar to those supplied by Siemens.

Also on review with Westinghouse and Brown Boveri, as of the latter partof 1969, it came out and was subsequently confirmed that in the case ofboth of these turbine suppliers, well worked out turbine control systemdesigns of that date were such as to make it possible to quote onproviding feed type forward signal controlled partial control valveclosure executed in a fraction of a second, while Brown Boveri was alsoprepared to ofier feed forward controlled partial intercepting valveclosure.

While the controls provided by European turbine producers cited justabove were of a type that required them to be preset prior to adisturbance, it will be apparent to those skilled in the art that therewould be no problem in providing a means for modulating such controls sothat the effects produced would become responsive to the magnitude of anelectrical signal since this would merely mean providing what GE (22,53) and Westinghouse (8, 29, 49, 50) either had already done and whichwould at most require employment of fast acting electrohydraulic servosystems of types such as were widely employed by the military forces ofthe US. and other nations during and following World War II.

Thus, as of and before the date of file of the application of the parentpatent there were several steam turbine producers that supplied turbinecontrol systems that were already adapted to, or could be easily somodified as, to effect partial as well as full closure of either or bothcontrol and intercepting valves in a fraction of a second on a feedforward basis in response to an electrical signal, hence with a speed ofresponse adapted to allow favorably efiecting system stability.

In addition it would not be difficult to show that it is old in the artof power distribution system switchgear design and control to providefor fast opening of circuit breakers of distribution circuits thatcontrol supply of power to the premises of customers, in as little as 2or 3 cycles, and to reclose if desired in a roughly comparable timeperiod, and it could further be readily shown that it has long beenfeasible to arrange to operate circuit breakers by remote control from acentral point and to transmit load data to control centers from remotepoints and to alter the setting of local control systems from centrallylocated control centers.

On this score the content of reference (52) well indicates that at thetime when the parent application was filed it did not represent aproblem that could not be readily dealt with to control load sheddingand restoration from centrally located control centers.

Therefore it appears evident that the aspect of the means or hardwareneeded to allow embodiment of the invention of the present applicationas a new method of power system control was either fully available or atany rate capable of being readily made available when the parentapplication was filed.

While the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand.

Thus, for example, persons skilled in the art can readily devise adigital type signal and memory unit for use in place of the analog typesillustrated in FIGS. 2 and 5.

Also, an aspect of a digital signal generator would be that values of Por in the event of an excess of power supply in system 1, a quantitywhich could be designated P Where L =electrical braking load P =ameasure of benefit equivalent to application of braking load that can bederived from reduction of system 1 prime mover driving power pluseffects due to fast regulation of power supply over D.C. lines, if anycould be arrived at in relation to type of power interruption andtransmission system load conditions by reference to suitable digitallystored data which could have been arrived at by suitable advance systemstudies.

Again whereas FIG. I shows a single circuit breaker at the terminus ofeach transmission circuit and serially connected in the output circuitof each generator, as will be understood by those skilled in the art,for the most part in present practice transmission circuits terminateat, and also generators connect to, a pair of cincuit breakers whichmake connection to each of a pair of basses, it will be apparent thatthe teachings of the parent patent and of this disclosure apply,regardless of whether or not bus connections are of the dual type.

Yet another point that may warrant note is that whereas in my patentsUS. 3,051,842 and Re 26,571 I refer to "segments of a power system" Ihave elected to also sometimes employ in the present and its parentapplication the equivalent expression "area of a power system, based onthe fact that the word "area" is more in conformity with usage of powersystem engineers Modifications and variations such as the foregoing areconsidered to be within the purview and scope of the invention and theappended claims.

In the matter of claim terminology the terms "fast prime mover drivingpower boost and "fast boost of the driving power of the prime mover of agenerator on a feed forward basis are to be interpreted as comprisingany process which causes the driving power of a power system generatorprime mover to increase in response to receipt of a feed forward typesignal and to do so rapidly enough to prevent or oppose development ofsystem instability in a period immediately following a stability endangering event in greater degree than would apply in the event ofreliance solely on the operation of a feed back type speed responsiveturbine control system.

.Slimilarly the terms fast prime mover driving power reduction and"efiecting fast reduction of the driving power of a prime mover of agenerator on a feed forward basis are to be interpreted as comprisingany process which causes the driving power of a power system generatorprime mover to decrease in response to receipt of a feed forward typesignal and to do so rapidly enough to prevent or oppose development ofsystem instability in a period immediately following a stabilityendangering event in greater degree than would apply in the event ofreliance solely on the operation of a feed back type speed responsiveturbine control system.

The term "internal path of power flow is used in the claims in a mannerthat is intended to be essentially selfexplanatory. However to clarifyits meaning, in FIG. 1 the radial path of power flow from generator G tobus I comprises a generator transformer and a circuit breaker, and thesame applies to generator G Also in the case of generators G and G thereare radial paths of power flow which relate to each generator andconsist of the generator transformers and circuit breaker but inaddition there is a further radial path of power flow which consists ofthe totality of transmission circuits l-la which tie bus 1-1a to bus 1.

If the generator G breaker opens as result of some event this willconstitute interruption of a radial path of power flow and the efiectwill be a loss of power input to bus 1 equal to the pre-event poweroutput of that generator.

If only one circuit of transmission system I-Ia opens this will notconstitute interruption of a radial path of power flow. However if allcircuits of transmission system 1-Ia open, due to any event, the effectconstitutes a sudden interruption of a radial path of power flow, whichcauses reduction of power supply to bus I, in the amount of the sum ofthe pre-event power output of generators G and G As explained in thesummary of the invention a fault on one line or circuit of atransmission system could sometimes have the eflect of causing one or mre generators to pull out of step with the balance of the power system,in which case all lines or circuits of the transmission system wouldordinarily open in response to protective relay system action and thistype of thing could operate to cause entire interruption of power flowover transmission system I-Ia which when happening would represent an"interruption of power flow to bus 1 over a radial type internal path offlow."

Similar considerations apply to generator G anrz the load connected tobus 1b, in that the generator breaker could open, or a breaker orbreakers controlling flow of power out of bus 1b to its load could open,and the efiect would be to either interrupt supply to, or acceptance ofload by, bus 1b, and would lead to a related change in the new powerinput to bus 1,.but it would also be possible for all lines or circuitsof transmission system 1-1 b to open, and this would constituteinterruption of a radial path of power flow that would reduce the powerinput to bus I by an amount equal to the algebraic value of thedifference of the magnitude of the pre-vent generator 6; power outputand bus 1b load.

In the body of the application reference is made to systems I, 2, 3, and4, which are shown diagrammatically in FIG. I and which may typically bedynamically relatively independent, or "integral, in that the rotors ofthe generators of each system can be expected to swing in the event of afault on either of transmissions 1-2, 1-3, or 1-4.

In the claims the word area is to be interpreted as embracing anyportion of a system or interconnection of systems which tends to behavein a generally dynamically independent or integral manner in the eventof a fault on a transmission circuit or some other system stabilityendangering event.

In the claims the words inter-area path of power flow is intended toinclude one or more circuits which constitute the total of circuitswhich connect system 1 bus to one or more dynamically integral areas ofthe total system, and the word inter-area transmission circuit isintended to mean any individual circuit that makes such connection.

Also the word system as used in the claims is to be interpreted toinclude an interconnection of systems.

Further, in the claims the phase "responding to the occurrence of atleast one type of event coming within a class of events comprising typesthat are adapted to cause interruption of supply of power orinterruption of power flow or power system instability is intended tomean response to operation of a relay or any other device that respondsto an event that merely may sometimes but would not usually bring aboutinterruption or instability, as well as response to a relay or devicewhich would normally but, in the case of some equipment malfunction,might not bring about the eflect specified, and where interruption isinvolved is intended to also include response to devices that respond tothe fact that a process of interruption either has been initiated or hasin fact taken place, such as circuit breaker auxiliary contacts that arearranged to either close or open in response to breaker operation, andrelays or devices that are responsive to interruption of current flow,and where instability is involved, to include resp nse to relays ordevices that respond to the fact that development of instability isabout to take place or has already taken place.

The term "system conditions is to mean station and line loadings, poweroutput of generators, directi n of power flow over lines, and lines andgenerators out of service.

The phrases "only when needed and "to whatever extent needed are to beinterpreted as representing something that it would be sought toaccomplish, if only imperfectly, but that would in any event be soexecuted as to provide a margin of safety whereby to assure againstdevelopment of system instability.

The phrase shed load" is intended to include shedding load at one ormore points within the area within which prime mover driving p werboosting is effected, and the phrase "application of braking load isintended to include application of braking load at one or more pointswithin the area within which prime mover driving power is effected.

Based on the foregoing definitions of claim terminology what [What] Iclaim as new and desire to secure by Letters Patent of the United Statesis:

[1. In a power system having generators and loads disposed upon thepremises of a customer and including a combination of interruptible andnoninterruptible loads, load shedding apparatus comprising means forreflecting interruption of flow of power from said system to saidinterruptible loads in response to momentary interruption of power fiowfrom said system to said combination of loads, and means forautomatically reconnecting said interruptible loads after a time delayperiod determined by a timing device] 2. In an extended power systeminterconnection incorporating power generating and power receivingelements, a transmission system comprising at least two lines which actin parallel to electrically unite system power generating and powerreceiving elements one of said lines being a DC. line, a device forreducing the power flow over said D.C. line in response to theoccurrence of certain events, control means for said device adapted forinducing power flow reducing operation of said device, means foractivating said control means, fast prime mover driving power and systemconnected electrical load control means for favorably affecting thestability of the interconnection notwithstanding the reduction of powerflow over said DC. line, and means responsive to the event leading topower flow reduction for initiating action of said fast prime moverdriving power and system connected electrical load control meanssubstantially concidentally with the occurrence of said reduction.

3. In an area of a power system within which power is distributed overpower distribution circuits to loads located upon the premises of one ormore customers, and wherein at least a portion of the load of at leastone customer is equipped with voltage responsive control means of a typeadapted to disconnect the said portion of the said load in response to amomentary interruption of power supply to said premises and toautomatically reconnect the said portion of said load on restoration ofpower supply, and wherein it is arranged so that restoration takes placeafter a preset time delay period determined by a timing device, andwherein, also power flow interruption means are provided which areadapted to momentarily interrupt power flow to said premises of saidcustomer in response to a load shed signal, the method of improvingreliability of bulk power supply within the power system which comsistsin the steps of,

(a) providing so that receipt of a load shed signal by said power flowinterruption means will cause the said interruption means to momentarilyinterrupt power flow to said customer's premises for a period longenough to cause disconnection of the said portion of said customersload,

(b) providing so that a load shed signal is automatically transmitted to'said power flow interruption means in response to the sudden occurrenceof one or more events.

4. The method of claim 3 wherein a load shed signal is transmitted tosaid power flow interruption means in response to the occurrence of atleast one even of a type coming within a class of events that areadapted to cause power system instability.

5. The method 3 wherein a load shed signal is transmitted to said powerflow interruption means in response to the occurrence of at least oneevent of a type coming within a class of events that are adapted tocause sudden isolation of a source of system power.

6. The method of claim 3 wherein the timing devices that controlduration of load shedding of those portions of system load that arearranged to be disconnected for preset time periods in response tomomentary interruptions of power supply to customers premises arearranged to be preset in such manner that when power supply is re--stored following momentary interruption of power supply loads that havebeen disconnected are reenergized progressively.

7. In a power system which includes a plurality of transmission circuitswhich make connection at each end to one of a plurality of transmissioncircuit buisses through power flow interruption systems comprising powerflow interruption means conjoined with power flow interruption controlmeans which are adapted to control operation of said power flowinterruption means in response to the occurrence of one or more events,a plurality of prime mover driven generators each of which also makesconnection to at least one of the said transmission circuit bussesthrough a power flow interruption system incorporating power flowinterruption means also conjoined with power flow interruption controlmeans that respond to one or more events, and wherein each generator isdriven by a prime mover which is provided with a prime mover drivingpower controller, an area of the system which comprises a transmissioncircuit bus to which loads and two or more generators which are internalto the area make connection over one or more internal paths of powerflow which are radial to the said bus in the sense that connection ofthe said loads and generators to the balance of the said system takesplace exclusively over one or more inter-area type transmission circuitswhich terminate at said bus, the method of design and operation of thepower system which seeks to preserve system stability via employment ofthe steps of (a) equipping the prime mover of at least one generatorlocated within the said area with control means adapted to alloweflecting fast boost of the driving power of the prime mover of saidgenerator on a feed forward basis in response to the reception of a fastdriving power boost signal,

(b) providing so as to cause transmission of a fast driving power boostsignal to said control means of said prime mover in response to theoccurrence of at least one type of event coming within a class of eventscomprising types that are adapted to cause interruption of supply ofpower to said transmission circuit bus by bringing about interruption ofpower flow over one or more of said radial type internal path; of flow.

8. The method of claim 7 supplemented by steps directed to minimize needto effect prime mover driving power boost as follows:

(a) providing a signal generator which generates and stores in one ormore memory devices, fast prime mover driving power boost signals themagnitude of which depends partly on the nature of any of one or moreevents that are capable of bringing about initiation of fast boosting ofthe driving power of the prime mover of the said generator and partly onthe magnitude and direction of predisturbance system conditions, andwherein the signal or signals are caused to be of such magnitude as tobring into efiect boosting of the driving power of the said prime moverwhen but only when needed as a way to provide against hazard ofdevelopment of system instability,

(b) providing so that the magnitude of the driving power boost signalthat is transmitted to the said control means of the said prime moverconforms to the magnitude of the stored signal that identifies with theprime mover driving power boost signal initiating event.

9. The method of claim 7 supplemented by the steps of (a) providing loadshedding means to eflect shedding of load in response to a load shedsignal (b) providing so as to transmit a load shed signal by the saidload shedding means coincidentally with transmission of a fast primemover driving power boost signal to said control means of said primemover.

10. The method of claim 9 supplemented by steps directed to minimizingneed to shed load as follows:

(a) providing a signal generator which generates and stores in one ormore memory devices, load shed signals the magnitude of which dependspartly on the nature of any of one or more events that are capable ofbringing about initiation of fast boosting of the driving power of theprime mover 0f the said generator and partly on the magnitude anddirection of predisturbance system conditions, and wherein the signal orsignals are caused to be of such magnitude as to bring into effect loadshedding only to whatever extent needed as a way to provide againsthazard of development of system instability.

(b) providing so that the magnitude of the load shed signal that istransmitted to the said load shedding means conforms to the magnitude ofthe stored signal that identifies with the prime mover driving powerboost signal initiating event.

11. In a power system which includes a plurality of transmissioncircuits which make connection at each end to one of a plurality oftransmission circuit busses through power flow interruption systemscomprising power flow interruption means conjoined with p wer flowinterruption control m ans which are adapted to control operation ofsaid power flow interruption means in response to the occurrencle of oneor more events, a plurality of prime mover driven generators each ofwhich also makes connection to at least one of the said transmissioncircuit busses through a power flow interruption system incorporatingpower fl w interruption means also conjoined with power flowinterruption control means that respond to one or more events, andwherein each genierator is driven by a prime mover which is providedwith a prime mover driving power contr ller, an area of the system whichcomprises a transmission circuit bus to which loads and two or moregenerators which are internal to the area make connection over one ormore internal paths of power Yow which are radial to the said bus in thes nse that connection of the said ioads and generators to the balance ofthe said system takes place exclusively over one or more inter-area typetransmission circuits which terminate at said bus, the method of designand operation of the power system which seeks to preserve systemstability via employment of the steps of (a) equipping the primer moverof at least one generator located within the said area with controlmeans adapted to allow effecting fast boost of the driving power of theprime m ver of said generator on a feed forward basis in response to thereception of a fast driving power boost signal,

(b) providing so as to cause transmission of a fast driving power boostsignal to said control means of said prime mover in response to theoccurrence of at least one type of ev nt coming within a class of eventscomprising types that are adapted to cause interruption of power flow tsaid transmission circuit bus over an interarea path of power flow whichincorporates at least a pair of interarea transmission circuits.

12. The method of claim 11 supplemented by steps directed to minimizeneed to eflect prime mover driving ower b ost as follows:

(a) providing a signal generator which generates and stores in one ormore memory devices, fast prime mover driving power boost signals themagnitude of which depends partly on the nature of any of one or m reevents that are capable of bringing about initiation of fast boosting ofthe driving power of the prime mover of the said generator and partly onthe magnitude and direction of predisturbance system conditions, andwherein the signal or signals are caused to be f such magnitude as tobring into effect boosting of the driving power of the said prime moverwh n but only when needed as a way to provide against hazard ofdevelopment of system instability,

(b) providing so that the magnitude of the driving power bo st signalthat is transmitted to the said control means of the said prime moverconforms to the magnitude of the stored signal that identifies with theprime mover driving power boost signal initiating event.

13. The method of claim 11 supplemented by the steps (a) pr viding loadshedding means to effect shedding of load in response to a load shedsignal (b) providing so as to transmit a load shed signal by the saidload shedding means coincid ntally with transmission of a fast primemover driving power boost signal to said c ntrol means of said primemover.

14. The method of claim 11 supplemented by steps directed to minimizingneed to shed load as follows:

(a) providing a signal generator which generates and stores in one ormore memory devices, load shed signals the magnitude of which dependspartly on the nature of any of one or more events that are capable ofbringing about initiation of fast boosting of the driving power of theprime mover of the said generator and partly on the magnitude anddirection of predisturbance system conditions, and wherein the signal orsignals are caused to be of such magnitude as to bring into effect loadshedding only to whatever extent needed as a way to provide againsthazard of development of system instability,

(b) providing so that the magnitude of the load shed signal that istransmitted to the said load shedding means conforms to the magnitude ofthe stored signal that identifies with the prime mover driving powerboost signal initiating event.

15. In a power system which includes a plurality of transmissioncircuits which make connection at each end to one of a plurality oftransmission circuit busses through power flow interruption systemscomprising power flow interruption means conjoined with power flowinterruption control means which are adapted to control operation ofsaid power flow interruption means in response to the occurrence of oneor more events, a plurality of prime mover driven generators each ofwhich also makes connection to at least one of the said transmissioncircuit busses through a power flow interruption system incorporatingpower flow interruption means also conjoined with power flowinterruption control means that respond to one or more events, andwherein each generator is driven by a prime mover which is provided witha prime mover driving power controller, and area of the system whichcomprises a transmission circuit bus to which one or more generatorsmake connection over one or more paths of power flow which are radial tothe said bus in the sense that connection of the said loads andgenerators to the balance of the said system takes place exclusivelyover two or more inter-area type paths of power flow which terminate atsaid bus and which include a total of at least three inter-areatransmission circuits, the method of design and operation of the powersystem which seeks to preserve system stability via employment of thesteps of (a) equipping at least one generator located within the saidarea with control means adapted to allow effecting a fast reduction ofthe driving power of the prime mover of said generator on a feed forwardbasis in response to the generation of a fast driving power reductionsignal,

(b) providing so as to cause transmission of a fast driving powerreduction signal to said control means of said prime mover in responseto the occurrence of at least one type of event coming within a class ofevents comprising types that are adapted to cause interruption of supplyof power to said transmission circuit bus by bringing about interruptionof power flow over one or more of said radial type internal paths offlow.

16. The method of claim 15 supplemented by steps directed to minimizeneed to effect prime mover driving power reduction as follows:

(a) providing a signal generator which generates and stores in one ormore memory devices, fast prime mover driving power reduction signalsthe magnitude of which depends partly on the nature of any of one ormore events that are capable of bringing about initiation of fastreduction of the driving power of the prime mover of the said generatorand partly on the magnitude and direction of predisturbance systemconditions, and wherein the signal or signals are caused to be of suchmagnitude as to bring into eflect reduction of the driving power of thesaid prime mover when but only when needed as a way to provide againsthazard of development of system instability (b) providing so that themagnitude of the driving power reduction signal that is transmited tothe said control means of the said prime mover conforms to the magnitudeof the stored signal that identifies with 35 the prime mover drivingpower reduction signal initiating event.

17. The method of claim 15 supplemented by the steps of (a) providingmeans for momentary application of braking load in response to a brakingload application signal (b) providing so as to transmit a braking loadapplication signal to the said means for momentary application ofbraking load coincidentally with transmission of a fast prime moverdriving power reduction signal to said control means of said primemover.

18. The method of claim 15 supplemented by steps directed to minimizeneed to eflect prime mover driving power reduction as follows:

(a) providing a signal generator which generates and stores in one ormore memory devices, fast prime mover driving power reduction signalsthe magnitude of which depends partly on the nature of any of one ormore events that are capable of bringing about initiation of fastreduction of the driving power of the prime mover of the said generatorand partly on the magnitude and direction of predisturbance systemconditions, and wherein the signal or signals are caused to be of suchmagnitude as to bring into effect reduction of the driving power of thesaid prime mover only to whatever extent needed as a way to provideagainst hazard of development of system instability,

(b) providing so that the magnitude of the driving power reductionsignal that is transmitted to the said control means of the said primemover conforms to the magnitude of the stored signal that identifieswith the prime mover driving power reduction signal initiating event.

19. The method of claim 15 supplemented by steps directed to minimizingneed to apply braking load as follows:

(a) providing a signal generator which generates and stores in one ormore memory devices braking load application signals the mangitude ofwhich depends partly on the nature of any of one or more events that arecapable of bringing about initiation of fast reduction of the drivingpower of the prime mover of the said generator and partly on themagnitude and direction of predisturbance system conditions, and whereinthe signal or signals are caused to be of such magnitude as to bringinto effect application of braking load only to whatever extent neededas a way to provide against hazard of development of system instability,

(b) providing so that the magnitude of the braking load applicationsignal that is transmitted to the said load shedding means conforms tothe magnitude of the stored signal that identifies with the prime moverdriving power reduction signal initiating event,

20. In a power system which includes a plurality of transmissioncircuits which make connection at each end to one of a plurality oftransmission circuit basses through power flow interruption systemscomprising power flow interruption means conjoined with power flowinterruption means which are adapted to control operation of said powerflow interruption means in response to the occurrence of one or moreevents, a plurality of prime mover driven generators each of which alsomakes connection to at least one of the said transmission circuit bassesthrough a power flow interruption system incorporating power flowinterruption means also conjoined with power flow interruption controlmeans that respond to one or more events, and wherein each generator isdriven by a prime mover which is provided with a prime mover drivingpower controller, an area of the system which comprises a transmissioncircuit bus to which one or more generators make connection over one ormore paths of power flow which are radial to the said bus in the sensethat connection of the said loads and generators to the balance of thesaid system takes palce exclusively over two or more inter-area typepaths of power flow which terminat at said bus and which include a totalof at least three inter-area transmission circuits, the method of designand operation of the power system which seeks to preserve systemstability via employment of the steps of (a) equipping at least onegenerator located within the said area with control means adapted toallow efiecting a fast reduction of the driving power of the prime moverof said generator on a feed forward basis in response to the generationof a fast driving power reduction signal,

(b) providing so as to cause transmission of a fast driving powerreduction signal to said control means of said prime mover in responseto the occurrence of at least one type of event coming within a class ofevents comprising types that are adapted to cause interruption of powerflow away from transmission circuit bus over an inter-area path of powerflow which incorporates at least a pair of inter-area transmissioncircuits.

21. The method of claim 20 supplemented by steps directed to minimizeneed to effect prime mover driving power reduction as follows:

(a) providing a signal generator which generates and stores in one ormore memory devices, fast prime mover driving power reduction signalsthe magnitude of which depends partly on the nature of any of one ormore events that are capable of bringing about initiation of fastreduction of the driving power of the prime mover of the said generatorand partly on the magnitude and direction of predisturbance systemconditions, and wherein the signal or signals are caused to be of suchmagnitude as to bring into eflect reduction of the driving power of thesaid prime mover when but only when needed as a way to provide againsthazard of development of system instability,

(b) providing so that the magnitude of the driving power reductionsignal that is transmitted to the said control means of the said primemover conforms to the magnitude of the stored signal that identifieswith the prime mover driving power reduction signal initiatmg event.

f22. The method of claim 20 supplemented by the steps 0 (a) providingmeans for momentary application of braking load in response to a brakingload application signal,

(b) providing so as to transmit a braking load application signal to thesaid means for momentary application of braking load coincidentally withtransmission of a fast prime mover driving power reduction signal tosaid control means of said prime mover.

23. The method of claim 20 supplemented by steps directed to minimizeneed to efiect prime mover driving power reduction as follows:

(a) providing a signal generator which generates and stores in one ormore memory devices, fast prime mover driving power reduction signalsthe magnitude of which depends partly on the nature of any of one ormore events that are capable of bringing about initiation of fastreduction of the driving power of the prime mover of the said generatorand partly on the magnitude and direction of predisturbance systemconditions, and wherein the signal or signals are caused to be of suchmagnitude as to bring into effect reduction of the driving power of thesaid prime mover only to whatever extent needed as a way to provideagainst hazard of development of system in,- stability.

(b) providing so that the magnitude of the driving power reductionsignal that is transmitted to the said control means of the said primemover conforms to the magnitude of the stored signal that identifieswith the prime mover driving power reduction signal initi- 38 (b)providing so that the magnitude of the braking load application signalthat is transmitted to the said load shedding means conforms to themagnitude of the stored signal that identifies with the prime moverdriving power reduction signal initiating event.

ating event. 24. The method of claim 20 supplemented by steps directedto minimizing need to apply braking load as follows:

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original (a) providinga signal generator which generates and 10 atent.

stores in one or more memory devices braking load UNITED STATES PATENTSapplication signals the magnitude of which depends 1,705,688 5/1929Staege 307 52 UX partly on the nature of any of one or more events1,935,292 11/1933 Griscom et a] 7 5 UK that are capable of bringingabout initiation of fast 7 0 9 19 6 Pope 307-49 reduction of the drivingpower of the prime mover 15 3,300 543 1/1967 R k flll t 307-86 X of thesaid generator and partly on the magnitude 3,124,699 3/1964 Kirchmayer30757 and direction of predisturbance system conditions, 3,229,110 I/1966 Kleinback et al 307-29 and wherein the signal or signals are causedto be of such magnitude as to bring into eflect application of HERMANHOHAUSER, Primary Examiner braking load only to whatever extent neededas a 20 U S cl XR way to provide against hazard of development of systeminstability, 30719, 29, 85, 102, 153; 235-15121

